Zach,
Why not rent two drift boat that are set up for singles in the front and the back. Four people in a boat is a huge pain. I see them from time to time on the Missouri and they are set up with two fisherman in the front and one in the back. The guides rowing these boats look miserable. In the guiding industry there is a $100 up charge to add that third seat and that is for a reason. It just plain sucks! I have only had to do it once and I had two little kids in the front of the boat and their dad was in the back. It was the longest day ever!
Just my thoughts but I would consider getting two boats instead of one just to make your experience a more enjoyable one.
Joel