I know your wanting a hub dynamo and they are nice but you might consider a bottle or BB type dynamo. There is a little more drag from these types when being used but the drag is not very much more and when disengaged there is no drag at all. If you commuted several hours every night a hub dynamo would be a worthwhile investment and I would not quibble about the price of a Schmidt. With Just an occasional use of the dynamo a bottle or BB type would do you fine. Considering it's going on a tandem you can use touring tires with bottle dynamo sidewall tracks molded in the rubber so sidewall damage from the dynamo wheel would not be a problem.