srw
It's a bit more complicated than that...
It depends on the tandem. One of ours is easily double the weight of a touring bike - and we're always the slowest up the hills.And tandems are great for steaming up hills. Think about it, they don't weigh that much more than a single bike (nowhere near double) - and you've got double the pushing power? And higher gears for powering down into a good fast cruise.
The other is a stone lighter. Over the last couple of days we've pulled away from riders, and overtaken them uphill - and not just with the benefit of momentum, either.