I think I read in one of Tandemgeeks posts that a tandems aerdynamics are not as good as you would think.
It seems to me that you are punching 1 long hole in the air friction category vs. 2 holes for 2 singles. You also have 2 tire frictions on tandem vs 4 on singles.
Can someone please chime in on this topic so that I can get some sleep tonight