My plan at the moment is to just ride my slowest heaviest bike with 2" expedition touring tires. If we go touring I'll just carry everything. That may be our ultimate solution. I'm not sold on a tandem yet, but I figure it's worth some investigation.
When my wife and I tour, I carry nearly everything in a 2-wheeled trailer, hoping it'd slow me down. It didn't work; the engine matters far more than the bike when it comes to speed. If you want to slow yourself down, 2" tires might help, but something like a Surly Pugsly with insanely wide tires might do the trick.