First, a little background:
My wife and I have been tandeming for about 2 months now, on a new Cannondale (medium/small). I'm 5'9, 147 lbs., she's 5'7" and about 130 lbs. (don't tell her I revealed these facts). We have the opposite style of you guys, I like to really spin, and she prefers a little slower cadence, but isn't a real masher either. We both 1/2 bike too, for a total of between 3,500-4,500 miles/yr. in mountainous terrain. We are really having fun with our new tandem and things are really clicking.
Thoughts:
All above comments are excellent. You'll probably be quite surprised the first time you both mount a tandem. It will feel like you're on top of a wimpy flag pole in a wind storm. Starting and stopping can move your heart rate up a few ticks, too. This will soon go away, and if you haven't ridden your 1/2 bike for awhile, you'll be surprised how squirley your 1/2 bike feels. For us now, our tandem feels more comfortable, and it's really great to have a stoker to visit with and watch out for traffic, etc. As noted, you'll suffer a bit on up-hill climbs, but the trip down is a gas! Although, I have to admit, we're almost as fast up-hill on the tandem as we are on our halves, but that's just starting to happen.
Your stoker may find the bars are too low with the standard set-up, but don't give up on that. There are solutions. You'll also find that tandems are special bikes, and not all accessories will easily fit. One thing I would encourage you to consider, should you purchase a tandem with Shimano STI brifters; Budget for a Flight Deck computer. You can't see the chain rings or the cassette (easily), and with the triple, there are lots of gears. You'll find yourself shifting a lot compared to a half bike.
Other helpful or fun items we have found we enjoy include a stoker mirror mounted on the bull horn (stoker) bars, a GPS for the stoker, a Topeak quick-release (small) front handle bar bag, a small seat pack on the captain's seat for the stoker's goodies, and a trunk bag for longer rides (or picnic supplies).
I guess you can tell, we're jazzed over our tandem. Be patient, remember that the stoker is never wrong, and enjoy. We're headed off to Las Vegas tomorrow to ride in the 'Vegas Century. Anyone else going???
OHB