Yeah, I came to the same conclusion. It's fun to think about how you would upgrade an older tandem. But you can't just get some sti shifters and be done--you need a rear cassette, which needs a new hub, and you may as well get a whole new wheel. Then you think about those old cranks, and maybe adding v-brakes and pretty soon you're into it well over $1000. And it's still going to be a heavy old steel bike. Best just to ride it as-is. Save the upgrading for something that is closer to what you actually want.
I ended up finding a 2006 burley for not much more than the price of the Santana. No upgrades needed.