Who would criticized you for that? Get a third one if you can! 
Ok, for a third cab I am actually contemplating a driving cab for dedicated driving games!
Being you have a dedicated 4-player, I would just leave the control panel as-is and use that for the multi-player party games (TMNT, Gauntlet, NBA Jam, etc.) You may have to add a third or fourth button depending on the games, but little else. I think when things start to get added to a 4-player panel (fighter button layouts, spinners, trackballs, etc.) is when it gets all unwieldy. But you have the 2-player cabinet to add all that jazz if so desired.
There are broad answers for your other questions so you pretty much just need to read up a bit and narrow it down.
Congrats on the find and enjoy!
So you are reading my mind. I was thinking about a massive MAME undertaking with multiple controllers, spinners, trackball, etc but liked the idea of more a real looking game room. I have been searching the internet for months now trying to figure out the best way to approach this. Almost picked up a 4 slot Neo Geo cab with games for $300 but was beat to the punch by someone who lived closer. (Guy had so much interest it became first come, first served! I lived 2 hours away!

) I am a disabled veteran on a fixed income and finally have been able to purchase a home for my family which also means I finally have a dedicated area for these endeavors. So, the plan is as follows:
2 player cab: MAME it up with all of the old school games (mario, pac man, commando, double dragon, etc) and mod the cab to accommodate the extra buttons needed for fighters (Street Fighters, Mortal Kombats, Fatal Furies, Dark Stalkers, Marvel v. Capcom, etc)
4 player cab: MAME it up for all of the four player goodness out there! NBA Jam, TMNT, Gauntlet, Avengers, etc!
Driving Cab: So in my opinion the best way to play a driving game is to actually sit down and drive! My inspiration is found here
http://www.hyperspin-fe.com/forum/showthread.php?12744-Outrun-cab-build&p=117663#post117663 I have a line on a Cruiz'n Exotica cab for $100+/-. I thought this could serve as a great launching pad for this conversion!
So here are my current questions/concerns:
1. Finances: I cannot tackle all 3 at once so I need to plan accordingly. I am trying to figure out what would be the best plan of action for attacking this plan. I have a PC that can be used for one of the MAME builds at this time. Which one to start first. I have narrowed it down to either the 2 or 4 player cabs as these cabs offer the most interaction for my family.
2. I have several gaming systems in my home currently. I have considered modding one for a temporary operating system for the remaining cabs. For example, I could use an old Xbox running XBMC with a ton of emulators to stand in temporarily. I could also use one of these systems modded to run the driving cab. I could simply use an off the shelf steering wheel/pedal system for now until I have the money to actually build the cab out the way I want.
3. Where should I/ how do I search for other members that are in my area? I am not scared of trying things on my own from research and asking questions here on the forum but it's always nice to be able to sit down and pick someone's brain!
Any/all questions/comments/concerns/feedback is always welcomed and encouraged! Thank you in advance for any advice you may provide!