Fun? Did you say fun? My wife and I haven't been on our singles since getting our tandem in late July.
Now back to reality. It is probably a good idea to borrow or rent a tandem before buying one. My wife is perfectly happy to trust her safety to me. She has her responsibilities on the bike and I have mine and combined we are a happy team. Some folks may not be willing to let go of the control which is required of a good stoker. Some folks may not be willing follow the cardinal rule of tandem riding "The Stoker Is Never Wrong" which is required of a good captain. There's nothing wrong with that. It just means that a tandem is probably not the best idea to maintain a happy relationship.
But when it clicks, I can personally attest that nothing is more fulfilling then being part of a working tandem team. ;-)