I haven't seen this discussed in recent threads so I though I would bring this up.
My wife and I have been tandeming for about 6 months and we absolutely love it! It's has really enabled us to spend quality time together (without the kids) doing something we both love (biking) but accomodates our unequal abilities.
Anyway, on the advice of another couple who tandem a lot, we got our hands on an intercom device made especially for tandem riders and we used it for the first time this weekend. At first, my wife was skeptical--"I don't like being plugged in to that thing" but after the first 15 minutes we both agreed that it really enahnced the experience. First, we didn't have to shout over wind noise, traffic, etc--so we could communicate important signals clearly and unambiguously. It also made it possible to have a constant dialog about traffic, road conditions, etc which made the ride much more safe. The best part was that we could talk about all sorts of other things that we often don't have time to talk about.
It kind of made the whole experience more intimate. We highly recommend it as a necessary part of a tandem team's equipment.
The device we used is called Tandem Talk, made by a little compnay in Ohio. There may be other similar products, but this was the first I came upon that was clearly made just for tandem riders.
I hope this is useful information for any other teams that want to better communicate on the bike.