One thing that causes decreased performance for tandem teams is a lack of syncronicity of efforts. If I am riding at "80%" (of, say, LTP) and my stoker is at 80%, then we are doing well. But suppose we're both at 80%, and we start to slip off the pack on a hill. I ramp up my effort to keep up with the pack, but my stoker doesn't feel any urgency and keeps the 80% pace. So I'm at 120%, stoker is still at 80%, and we're climbing like we are both at 100%. Except I'm wearing myself out a lot faster. If on another hill, I'm riding up at 80% and my stoker is at 120%, again, stoker is wearing him/herself out faster.