Well, FWiW there are many old French tandems from Gitane, Motobécane, Peugeot, and The like. Given that they are French (horrors!) and old, they can frequently be found for dirt cheap. I have a 1971 Gitane tandem that I am rebuilding now. I plan to use it only on flat, uncrowded roads so it will be fine. But something similar could give you a taste for what tandeming could be.