���Build two. Tandems are also called divorciacks.

Canuck's advise is the best.  Build a tandem for the two of you and a single for yourself. 

If you can't do that and are determined to have one boat to use as both a tandem and a single, I'd recommend one of the tandems that has a single large cockpit like the Wood Duck Double, Mill Creek 16.5 or Microbootlegger Double.  With all three, you could add a third seat in the center of the boat for use when you paddle it alone.  Of the three, I beleive that the Microbootlegger would be the best choice.   

Not sure if you have ever paddled a tandem, but as Canuck says, some call them divorce boats.  On most tandems, the seats are close enough together that the two paddlers have to be disciplined regarding their stroke to avoid hitting paddles.  All three of the boats above fall into that category.

We selected the Shearwater Double because the cockpits are 6.5' apart.  This allows both paddlers to paddle at their own pace with not chance of conflict.  As you can see from the picture below, my bride takes her paddling very seriously.  We have had ours for 4 years and absolutely love it.  I suppose that I could use it solo if I added some ballast up front, but for solo paddling I prefer a solo boat.  As a double, this is a wonderful boat.  It is fast, stable and hauls a good load.  My wife has a couple of great singles, but this is the boat that she uses the most. 

   Hm, Thanks Canuck                                                                                                             I never considered the knocking paddles.  That's one to think about.  I expect my wife to be 130 pounds of not much help.  I imagine that makes getting around a little more difficult and a lot more tiring.

I do already have my Wood Duck 12.  So the Tandem will be a second boat. I guess I'll need to bring'em both on long trips.

Thanks always

Bri