OP; Recommend check on Nova Cycles (see builder suppliers section). If you search there using "tandem" as a search string, you will find a nice fork crown, chain stays, etc. Read the notes on using the tandem CS's as xHD fork blades. Quite a few double OS tubes are available also, but be sure you check the lengths to make sure it is long enough. I would think a 1 5/8" down tube would do the trick...or maybe be too big. Of course also check the thickness of the butts and the centers.

Hope that helps
/K