I have seen many studios with 2 sets of monitors. My current monitor set up is I have a pair of 1st gen (purchased in 2004) Event 20/20s (passive) hooked up to a crown XLS 2502 amplifier. My question is would anyone recommend getting another pair of studio monitors and if so why? Also if so what would you recommend? Thank you

I use three sets of speakers. It just gives you a great overall perspective on how your mix sounds.  It is better if the speakers are not similar as well.
 
My main speakers are Mackie HR824's on concrete stands and they have that smooth well balanced sound right down to very low frequencies as well.  Smooth highs. smooth mids nice low end. I use them the most.  
 
I recently came into possession of a pair of Yamaha HS7's.  My son moved overseas and left them for me so I decided to set them up too.  They only have a 7" woofer so to start with they just do not show up any deeper sub type bass in fact they are good at ignoring it.  These speakers have the white cones similar to the NS10's and in some ways they sound like them too.  Because for a start they are not pushing out heavy bass, the mids are just pushed forward naturally as are the highs too.  So they are great check for a mix at any time.  They certainly emphasise things the Mackie's do not.  It is interesting how you can satisfy both speakers after a while too by making some subtle adjustments on the HS7's I find the sound gets better on them but on the Mackie's things have not changed so much.
 
The third speaker I use is a single mono type Auratone type device being fed a L+R mix and it is great down at low volume.  Different to either of the others and will show up a whole draft of other issues the main one just being what I call the critical balance.  Mono compatibility etc.. Vocal balance.... Really the most important speaker I have!
 
It is important that the polarity of all speakers agrees (which they should anyway) so that if for any reason you do have them both or all on then they will all be in phase and sound good. You also need the means to be able to switch and drive them all with individual output level controls so that they can be matched SPL level wise too. A speaker switcher comes in handy here. Many monitor controllers offer multiple speaker output options.

In the big studios the standard set up is a big set of full range speakers hung on the front wall. Then you might see as much as 2 even 3 sets of near fields. The offering is because different people will be using the studio and everyone has a favourite. The studio needs to please the status quo.  The most common speaker siting there has always been the NMS10. Auratones are also a very common near-field. Now of course powered speakers are the norm and you'll see more of a variety. 
 
In a home studio there's not much point in 2 good quality near-fields. You really need to learn your monitors and having 2 sets will just water down your ability to learn and trust your monitors. 
Example, I have my NMS10's and a set of Tannoy PBM 6.5's. If you hook them both up the Tannoy's would win the overall sonic impact and full range sound shoot out hands down. But..my mixes are always accurate when I use the Yamaha's.
 
I used the Tannoy's for my first 10 years of recording and back then I made pretty good mixes. But it always took 6+ times from the car back to the studio to accomplish this. Within a few weeks of using the Yamaha's all that changed and I often get it right the first time. 
I still use the Tannoy's to listen to mixes but they are now in another room serving duty as a home stereo set.
 
I do have a second set of monitors rigged in the studio which are the equivalent of a computer speaker. I can A/B a mix with those but I never really learn much.  I also have a Sub which will make up for the Yamaha's pathetic low end. I only turn it on AFTER I'm happy with a mix. It certainly sounds better but is a dangerous tool to use full time while mixing.  
 
So bottom line, you can always purchase a "better" set of monitors and retire your old ones. But if you are happy with your mixes why spend the money. If your not happy with your mixes then yes, buy a new set but then why would you want to use the old ones?  Sell them and buy a nice chair.  
 

Indeed it is better to use a variety of sources for check up, then a singe reference monitor system
 

Two sets of near fields are also fine indeed as long as I mentioned they are different enough from each other.  In my case the Mackie HR824's and the Yamaha HS7's sound very different to each other.  So in that case the second set is still valuable and useful.
 
What would not be good is for the second near field set to be virtually identical in sound to your main speakers. For example there are some near fields that have a very close sound to the Mackie's and then you would not be gaining much. You can still do it on one set of speakers too but having the other is a bit of a luxury. 
 

Jeff Evans
 
The third speaker I use is a single mono type Auratone type device being fed a L+R mix and it is great down at low volume.  Different to either of the others and will show up a whole draft of other issues the main one just being what I call the critical balance.  Mono compatibility etc.. Vocal balance.... Really the most important speaker I have!
 
It is important that the polarity of all speakers agrees (which they should anyway) so that if for any reason you do have them both or all on then they will all be in phase and sound good.
 
You also need the means to be able to switch and drive them all with individual output level controls so that they can be matched SPL level wise too. A speaker switcher comes in handy here. Many monitor controllers offer multiple speaker output options.

Very good advice here on polarity, monitor controller and mono type auratone speakers. 
 
The auratones (plus the controller to switch quickly) are excellent value for what you have to invest (once you get into the habit of really using them because the mains will always sound so much 'nicer').  I do a lot of mixing and an EQing on them these days ... where I deviate somewhat from Jeff's practise is that I do not exclusively use them in mono mode because I quite appreciate what they also reveal in terms of panning and space e.g. placing instruments too close together and how they mask/colour effected parts.

For home studios most of people simply use the mains and then drag copies to other environments.  I listen to mixes on this (non music) computer's speakers, my jeep and living room book shelves.  But most pro studios offer near fields for their clients to also reference, while some engineers use their nears for most mixing and use the mains for check up.  As noted above, mains can give someone too good of a sound (when most users are listening on ear buds) and too loud when listening all day.
 
The little yamahas - msp5s, are often used for near fields.  Those would be my choice in the lower price range. Their HS series were designed for home use and pleasure rather than to be "revealing" (tho they are fine as alternatives).  But really, listening to real world alternatives (like in a car) will give you a good idea of product flaws and, unless you have a well-treated room, environments that don't share the same flaws despite using different speakers.
 
@ 
 
 

Since I have the 1st gen Event 20/20s how would I know which monitors differ to give me that A/B comparison. Currently I do go back and forth a lot from studio to smart device to car lololol. If getting a 2nd set can eliminate alot of the back n forth where do I sign? :)

Seems just having different speakers may not solve that issue. You still need to listen in different "spaces" 

i have 4 monitors - JBL Control One, Bose 901, M-Audio SB-5B (main), and a mono Auratone. the Bose are connected with a JBL sub. mainly begin mixing on the mono, then switch the SB-5B for stereo, and then test across Bose, SB-5B, and the Auratone. oddly enough, if you try to mix on the Bose, the mix generally doesn't translate well, but if you get a mix from the other monitors sounding good on the Bose, the mix translate really well. bizarre...