Hello, “Duck” and welcome to the forum.
I don’t think many of us here have experience with the two-holed mansion houses. There are pros and cons to using a two-holed box, or of hanging a box in a tree, for that matter. What are the reasons that are pointing you in that direction? That might help us to refine our recommendations.
Most of us use a standard NABS-approved box design with a single 1 1/2″ round hole, or the Peterson box. Some use a Gilbertson box, but those are getting really difficult to find.
Are you looking to make your own box, or purchase one ready-made?