I totally agree with the above posters who suggested not spending every minute together...or at least make sure it is understood that time apart is an option. Also, try to anticipate potential problems in advance based on the different personalities and try to head off conflicts before they start. For instance, is one person in your party an early riser, while someone else likes to sleep till noon? And what about touring styles, eating habits, etc?
We had a succesful 'group' trip back in 200 when when we went with my sister, my parents AND my in-laws (the parents and ILs joined us for the last 2 days). It was a recipe for disaster: My parents and in-laws have totally different personalities, my MIL and my FIL do not get along at all, my MIL is a complainer and very overbearing and can't tolerate the heat (we went in May), and to top it all off....my parents hate theme parks. A month in hell sounded preferable to that vacation, but it wound up being the best vacation we ever had.
I think what made a differance is that DH and I laid down some ground rules first. (Since it was 'our' vacation and they were tagging along we could do that....yours sounds more like a joint undertaking). That saved the day. With 1 exception (and really, it's totally unrealistic to expect my in-laws to get along for a full 48 hours) everything went SO much better than I ever expected. I got to share my favorite place in the world with the people I care about most with no conflicts, fighting or *****ing. That in itself is a miracle....I never would have believed it. So...it can be done and really, I think with some planning , WDW is so much fun to do with a group! I will treasure the memories of that trip for the rest of my life!
Good luck
Kim