Both my wife and I are overweight. We got into tandeming when my wife said, "We need to find something to do for exercise we can do together, like a bicycle built for two." So... we started looking. We decided that if we were going to do it we were going to do it. We bought a Trek T900, rack, helmets, bike garb, water bottles, cages, helmets, etc. etc. and have not regretted it.
If they are heavy make sure they get something sturdy. We are over 400 pounds, so I know of what I speak.
They sound like they will be a great team. They both want to do it. I think they will have a great time. Make sure they get to test ride it. They probably don't want a speedy, drop bar, skinny 700C tire to start with. But I jump to conclusions.
Good luck.