I spent a 2 weeks trip solo in a 17&1/2 Ft barge.  I started off paddling it backwards from the front seat and found the boat too wide to easily paddle.  I eventually settled on setting in the stern seat and putting gear as far forward as I could to balance the load. Many of the tandems are so wide that you will need a slider seat sliding right to left to get the paddle in the water on both sides of the boat(well maybe that is not a good ides)and (just joking).  As far as what boat to get I would start off looking at your tandem requirements and look for the smallest boat that would meet those requirements.  You will get a new respect for the wind.