You should start your own thread in the hardware forum where this belongs. 
I'll answer you there. 

We used to have the Hear system, the personal mixers always broke and sounded like sh*t. Without even testing them out we replaced our Hear system with the Behringer platform: http://www.behringer.com/EN/Category/Signal-Processors.aspx?s=H870
 
No regrets, whatsoever. A VAST improvement in ALL respects.

Been using the Hearback for many years, probably not too long after my earlier posts. Haven't had an issue with quality at all. No ones ever complained. Been very happy with the system.

There are some limitations as far as using as a descrete 8 channel individual mixer. Channels 1&2 are always stereo. Has not been a problem as I usually use 1&2 for drums. Channels 3&4, 5&6 and 7&8 are either straight up the middle or hard panned with the link button on for each set individually. Also, the limiter is a waste as it does not sound good. But really, the limiter is not needed.

Looks like Hear has a new system out, HearBack Pro and it sounds like the panning issues may have been taken care of. That's probably the unit I would have gotten if they had had it when I bought mine.

Overall, the Hearback has worked out very well. Has made tracking much easier as the guy working the board doesn't have to constantly tweak the headphone mix for the musicians. The musicians can do it themselves.

 
1. Is there a way to combine both interfaces so that i can use them together?
 
No, unless you use a separate computer for each.

2. The concept of sub mixes is one that I don't quite understand I think. Sonar I hear has the ability for sub mixes anyone care to explain that please?
 
A sub mix is just a send on the channel.  You can set up as many headphone mixes as you have physical outputs for.  You simply assign a stereo send to the tracking channels, dial in the amount of vol you want of that track's recording, and assign the send's output to a physical output that goes to your headphone distribution unit.  If you only have the one on your interface, then to there.

 
3. I can wrap my brain around individual instruments going to aux send out but how would you get an entire drum kit mic'd with 8 mics sent to one channel?
 
Each send is on the 8 tracks.  Each send controls the vol for that track in the send mix.  But, just send the overheads unless someone wants more "whatever" in their headphone, like kick.  Then simply add the mono source to the send.

4. The goal is for every musician to control their own mix I would assume I need all separate sends out of Sonar into what exactly?  I read for the hearback there is a card that has 8 analog inputs I am assuming I would connect those eight inputs into the analog outputs of Motu or Ocapture or ???

 
Each line output on your interface, other than the master output, goes to a headphone distribution unit.  It takes two line inputs and makes a stereo output, controlled by a fader (or knob).  A headphone amp (with several channel outs) is simply a line mixer that has the proper ins (mono 1/4 inch for line level) and outs (on stereo 1/4 inch plugs usually)for headphones.  You make the mix in Sonar, send it to the bass player's headphone output, and he/she controls the volume.  If they need to hear more kick drum, you can turn it up in Sonar.  It is the same way a major studio uses a physical board - one that has 8 or more send knobs.  Using the prefader (usually), you make a static mix for use downstream that is separate from the main mix, which you will still have in your main monitors.  The talent, like a singer, is hearing their vocal in the headphones with reverb, but you don't want that reverb recorded.
 
5. In Sonar does that mean each output on the tracks is not using the master audio output channel to one of the analog outs on my module?

 
NO - the main out still goes to the master out.  The headphone outs are completely different mixes (using the send knob instead of the fader) going out separate DAs and physical outputs.
 
5.I have heard the term use the aux input on the hearback system for more me, how is that possible let's say for a guitar player?  If his guitar is in the main DAW for recording what is used to send him back into the headphone system as an aux?
 
Say what?
 

I work in a home studio/writer/arranger/composer environment, as I suspect a lot of Sonar users do.  Good foldback is so important to the creative process I think it's really neglected compared to all the discussion about new software toys.  The great news is that hardware is now so cheap there are readily acessible solutions for those who can't justify/don't need the pro solutions discussed (very informatively) above.  For me, hardware is the way to go: no extra CPU load and independent control.  The quality of foldback sound isn't so critical as for the recorded signal, so cheaper stuff will do, and even the cheap stuff is pretty good.  I use a UK £99 6-into-2 mixer (Alto ZMX122fx), which is OK quality and has on-board Alesis effects, so a little reverb to juice up the vocals is right there (not recorded, of course).  I use a separate AKG head-worn mike and consumer headphones, and a blue-tak attached piezo bug (John Pearse, £34) for acoustic instruments.  All I need from Sonar is to insert a send to a stereo foldback bus for a backing/metronome signal running into 2 line inputs on my mixer.  I do use a VS-100 audio interface which has 3 pairs of audio outputs - but even with a stereo-out-only interface you could take the headphone-out and drive the foldback mixer with that.  Of course I still use large diaphragm condenser mikes for the actual recording.  The huge plus of this approach is that latency is just not an issue - and even with midi, I say get the notes down and play with the sounds afterwards.  Works for me!