I think it depends on your plan.Two seperate cookers on the same trailer can be nice. Maybe one a smoker, one a grill or rotisserie. As Frank siad,I'm going with one long unit that is split by an internal divider. It is going to be huge! My thought is either way will work, trailer size may dictate. I am using a 16' x 78" trailer. With the long unit I can get the balance I want, and have plenty of room to get around the cooker. (i will cook from ground) With two seperate floor space would be at a premium, especially trying to control tongue weight and balance.
The other obvious consideration is money. With two you can do one now and the other later, if need be.
Whatever you deicde have fun, and remember as soon as you are done you will think of something else. It just keeps going!
Cant wait to see what you do!