Talk a lot . . . and then talk some more. It usually works best for the stronger and heavier rider to be in front. It takes a lot of strength to hold up a tandem or control it in sudden manuevers.

What type of tandem are you renting? Just a townie or mtb to do some slower speed rambles might be good for the first time out since your DH isn't as experienced.

http://www.bikeaholics.org/CaptainTraining.html