One thing I'd like to add, without being too far off topic I hope, is that my wife and I are long time users of a wired communication system. These are available in a couple of flavors, "TandemTalk" being the one that most people have heard of. A system like this makes communication effortless, and since it is so easy, you communicate far more than you would without it.
The only real drawback I've found to using one of these systems is that you are
wired together, and that takes some getting used to, mainly when getting onto and off of the bike. It's well worth the effort. There is at least one wireless system available, and I've thought about trying one of those some time.
There are many benefits to being able to communicate this way- we used to do a lot of what I would call "competitive pack riding", and when things get wild and wooly, its nice to be able to tell your stoker to get ready to sprint for the county line sign (or whatever) without having to yell it over your shoulder at 30mph and alert the rest of the crowd to be ready to grab your wheel for a leadout.
Or: If you're on really
good terms with your stoker, you will sometimes find yourself involved in quiet and very interesting discussions of things that might happen after the ride
At any rate, after having used one of these things for years, I'm really surprised that I don't see more tandem teams equipped this way, and I can highly recommend using one. Neither I nor Terri would consider riding without our intercom unless there was some very good reason for it (like battery crapout).
The two brands I am aware of are TandemTalk and TandemComm- I've used both, and have a preference for the TandemComm, mainly because it is a smaller unit and can use rechargeable AAA batteries. Conversely, I prefer the headsets that come with the TandemTalk.