Hi Michael, I have installed double doors made from two door slabs and it worked great. Commercial jobs are almost always done this way. To figure your rough opening, add twice the thickness of your jamb stock, the two door widths, and 7/16" additional for clearances. Add an additional inch for shimming, giving you a 1/2" each side between the jamb and the framing. This will allow the finished doors to have 1/8" gap at the hinges and 3/16" between them. Dedicate one door to be active and the other passive. Install flat door stop all the way around the jambs. Install a floor bolt and/or head bolt on the passive door and a flat door stop or mullion strip on the active door face, hanging slightly over the edge. When closed, it will will come to rest against the passive door. The passive door should be fitted with a latchplate and have a dummy handle. The active door will have an operating handset and will have full locking capabilities when the floor and headbolts are used to keep passive door closed. Make sure you secure the top of each jamb securely to the framing, at the top hinges, after shimming to keep the doors from sagging over time. Congratualtions on using your head to save a few bucks. Good Luck.