GH,
I've read a lot of posts equally divided between "weight training is good" and "weight training doesn't do a thing". After all, Lance didn't weight train for TdF so some say. My take is that someone on a structured, discipline cycling only training program could do darn well riding exclusively, but riding a tandem for strenuous recreation with an SO is not a structured training program. When you undertake a big climb you can't stress your muscles the way you do on a machine or with free weights because you're in survival mode.

Short story: I'm doing strength training - quads, hamstrings and about everywhere else. I've already noticed some improvement in leg strength and I think the upper body and core work will help even more on a tandem than on a single. I've got 20 years of sitting behind a desk, riding horses and moving hay bales to make up for.