1. Just to make sure, you've been through 2 crank arms and 4 bottom brackets?
2. Are you sure you have the right bottom bracket, IE Octalink V1 and V2 are not compatible in either direction. A V1 crank can be be forced onto a V2 spindle, but will fail in short order. If I remember correctly Ultegra is V1, and other than XTR, all of the MTB (with perhaps longer spindles?) versions are V2.
3. If the bike doesn't fit, I'd consider changing just for that. If your current frame did fit, replacing the cranks with a different type is always an option, though tandem parts aren't as easy to come by.