Here is what DaVinci puts on their tandems http://www.davincitandems.com/options.html
The smaller, higher pressure tire will roll better, but it will be more prone to pinch flats, cuts, and have a harsher ride. I'm assuming that tight handling is not a concern for your train. I would think a 28 or 32mm tire would be appropriate for you. I would suggest running at least the max pressure the tire is rated for. A Conti Gatorskin 28c is rated for 120 psi.
I don't know of any tires being tandem rated.