We went last year with my DH's mother, brother and sister-in-law. We had a 2 bdrm villa at OKW. I won't do this again, unless we are in seperate rooms and we just agree to meet somewhere in the middle of the day.
They expected us to plan every moment without any input from them. I would give them descriptions of things and offer choices and they would still say "whatever you think is best". I was pretty sure they didn't like some of our choices.
I suggested we split up when it was obvious to me that my sister-in-law was tired and not feeling well, but they insisted on all of us sticking together and toughing it out. I had a hard time enjoying myself knowing she wasn't feeling well, and all of us ended up cutting our day short, which made me cranky.
I think most of the problem was that they wanted to be polite, and didn't speak up about what they wanted to do. Make sure you talk to the other couple before you go about what a WDW vacation is to you and how you want to spend your days. Find the common ground between you, and share things you both enjoy, but be willing to go your seperate ways the rest of the time.
BTW, we just went on a cruise with Brother & Sister-in-law and had a fabulous time! We planned some things together, and spent some time apart. We can't wait to do it again.