After years of performing as a jazz pianist in a small group, I now find myself in a bues/rock band and in need of personal, in-ears monitoring. Upon the urging of my bandmates, I bought a Rolls PM351 personal monitor station. In theory, it allows you to monitor the signal from the entire band, plus isolate your vocals and instrument (s) and adjust the sound to your preferences without controlling the FOH mix. In trying to use this, there is only one instrument input, and I have a two keyboard setup. I haven't been able to successfully set it up so I can monitor and control my mix for both keyboards--I tried a y-cable, but instead of getting both signals I got one, and only in one ear.  I don't know if I can get a cable to merge the two keyboards (even if it's in mono) into that single instrument input, and if anyone out there has done so, or used a different (small) product, I'd love to know. I do have a Behringer mixer, but while it's somewhat compact, it's really a full-mixing board, and I would prefer not bringing it with me--not to mention I still have some issues obtaining the FOH mix and my personal mix independently--many of the venues are not set up to accommodate two keyboards, it seems.
 
Anyone with either experience with the Rolls PM351, or another product, please share with me your success. Thanks. 
 
PS--I'm old, so explain it to me like I'm a 5 year old and I will probably understand it better--:) 

Well I have the Rolls PM 55 which is the same thing minus the instrument input. 
Generally the way it works is your Vocals  go throught the XLR thru and the band monitor mix to the line in. In your case you also have the Instrument thru that would seem to work the same as the vocal thru. Whats missing for you obviously is you actually need another instrument channel. 
So other than buying a similar device with that opton would be a small sub mixer to combine your 2 keyboards... I don't recomend a Y cable as that can even damage equipment. 
 
An example of a small mixer would be a Mackie Mix 8 or even the 5 You need 2 stereo channels and stereo out.
https://www.sweetwater.com/store/detail/Mix5
 
 If your band does sound frome stage you don't need balanced outs. There is also a Radial DI made just for keyboards that would be perfect if you do use a snake. 
https://www.sweetwater.com/store/detail/KeyLargo
 
 
Most keyboard players I've worked with have a small sub mixer as this also free's up a channel on the FOH. 
 

Thanks, Johnny, you have confirmed my suspicion that the Rolls is just not the solution for me, I will likely sell it and get a smaller mixer to do the job. Thanks for the links....

I do have a follow-up question, however. Having looked at the Mackie Mix 5, it has the inputs I need for keys, and also a vocal input. I use a talk-box pedal for effects on my vocals.  I want to be able to hear the band in my in-ears, and adjust my instruments/vocals individual without affecting the live mix. That's what the Rolls 351 does well (if you have one instrument to plug in). I'm having some problems figuring the routing I would use (either to go into the snake at the venue) or to go into the FOH mixer (on small stages we often perform on). Any other suggestions/diagrams I could use (ps, it seems that the sound guys I work with at the venues don't always know what to do with my keys, either).  Thanks :)
 

A mixer is not always the solution because they don't have true unaltered thru put for the vocals and instruments.
 
The Rolls is my favorite solution but is lacking the extra input. 
You need  to split your vocal mike so just the unaltered signal goes to the FOH mixer. If you use a mixer all your vocals and instruments and the bands monitor mix will be sent to the FOH on one channel. Unless that mixer is overly complicated which will also mean it will be big and cumbersome. 
 
So the Rolls is the solution but you'll need to combine your 2 keyboards. Therefor any small mixer but from what you've described I would look at the Radial DI as that gives you the best quality output for your keys in any situation, It is the standard for live sound on big stages. The sound person gets 2 XLR feeds from you. Your vocals and the keyboards. 
 
I'll draw you a diagram later today when I get a chance. 

It sounds like the best idea might be to hook up everything to a small mixer then use an output from that to drive your in-ears.

So your keys and anything else you use to create sound connect to mixer inputs. You can then use the mixer’s main outs and auxiliary sends to set up whatever mix you require for your personal monitors and to go to FOH.

For example.

All keys connect to line-ins on the mixer, the vocal effects to a channel with a mic pre. The send from the main foldback system is also connected to a mixer input.

You then use the mixer’s main outs to send to the FOH mixer and use the channel faders to set up that mix, leaving the fader for the channel that’s receving the foldback mix turned off.

One pre-fader aux channel is then used to set up your headphone mix which you tap from the relevant aux send on the mixer. Another aux send is used to send your keyboard mix to the foldback system seo everyone else can hear it.

There are other ways of routing that can be equally valid, and may work better. Such as your foldback mix coming off the mixer’s mains so you can use the mixer’s eq on it and the pre-fade aux sends being used to send signal to the FOH and monitor mixes.

I hope that’s clear, it’s one of the things that’s easiest to show using a diagram.

The whole setup is best sorted out during practices, obviously. As a final thought, having an eq on your personal monitor feed can be very useful as well as in-ear monitoring is subject to the same sorts of issues as any earbud and the eq may need tweaking to stop everything in the earbuds sounding too bright or too bass heavy.

And watch out for feedback. “Proper” in ears that effectively seal the ear canal are pretty resistant to feedback but it can still happen, especially if you’re using a mic. It’s not a pleasant experience. Also, the in-ear mix might be best done in mono. Stereo in-ears can work, but if you move around much then it can really upset your sense of balance if the sound-field doesn’t shift around the way your brain expects. Vocalists can be really susceptible to that problem.

 
 

Tim  a mixer can work, but you would need a more advanced mixer that had the correct features. But I like your description which I hope the OP understands,, I did.
 
First for the keyboards
The Radail DI is perfect because it's optimized as a DI and can also send either summed mono or full stereo which is important for keyboards. Just sending one channel of a stereo keyboard can thrash the sound. 
Some keyboard do sport a Mono output but they are rare. There are lots of options for the keyboards including a keyboard stage amp with a DI output and aux send. A lot of powered speakes have built in 3 channel mixers and mixed outs. 
 
 The Vocals
Running a vocal mike into a sub mixer before it is sent to the FOH would freak out most FOH engineers. 
The Rolls uses a Tranformer isolated monitor send to the in ears while the XLR in and out are left original. I tested mine and there is zero differance between the input and output and absolutly no noise added. Pretty good stuff at the price point. So that alone is why you would use it for a peronal monitor set up. Mine is very simple and once I first used it I would never go back. there is nothing like hearing your voice that loud and with zero latency too!  This is hard wired of course, Wireless in ears would be a totally different set up as you would use the FOH or monitor board for your mix and not need any of this. 
I wear only one in ear and leave the other out so I hear my guitar amp live from the stage. The OP has that option as well. I like to hear the full spectrum as in ears are too tinny for instruments for me. The band I play in we only put the vocals in the monitors anyway. The band shares a wireless system one of those chepo Galaxy systems with 5 bodypacks. I didn't like it so went hardwired. The Rolls is right on my mike stand. 
 
So the whole system is up in the air depending on what you need in those in ears.. I put only the other vocals and then mine on top in the one ear. The rest of the band and my guitar I'm hearing normally like for the last 50 years :) 
 
 

wow, this is a lot to chew on, and I like having various options to mull over. Maybe my Rolls can be used after all, but lots of things to consider, and cost is also an issue (the DI box described is rather pricey, but has it's advantages, for sure. Thanks to all. Any diagrams would be lovely, too, thanks.

 
Here's an option like my set up only differance is I play guitar and I mike my amp to the FOH. With a Keyboard amp they normally have both a XLR DI and 1/4" line out.
So instead of patching the keyboards to the Rolls, you would use a live monitor to hear your keys and only hear your vocals and the band in the in ears.  
Ya it all costs money but if your planning on playing a festivals it's sort of nice to have a solid set up that doesn't change from stage to stage. I bought 2 Radial DI boxes this year just for festival set ups. A Tone Bone PZ pre for my guitars and a  Trim two box optimized for a laptop ( or any device like an iPad.)  So I'm set for the big time now :)  I don't use my in ears for those stages as they mostly have smoking monitor set ups.