+1 on the Conti GP 4 Season. We are a 350 lb team and have had good luck with these tires. The only issue was a brand new tire rolling off the rim. It turned out to be a tire bead defect and Conti replaced it free of charge.
Previous to the 4 Season we used Conti Ultra Gatorskins. This is a good durable tire but we had repeated issues with the sidewall threads coming off the tire. I ride the Gatorskins on my single and have not had the same issues. I believe the thread issue was caused by a combination of sidewall flex from the extra loads on the tandem as compared to a single, and the sharp edge on the rims of our Rolf Prima wheels. We are now doing most of our riding on Velocity Deep V's and I have noticed the edge is not as sharp as on the Rolf but I like the 4 Seasons and will continue to use them.