Originally Posted by cpach

What's the hub/frame spacing? Given this is a tandem increasing dish may result in an unacceptable tension differential. If your bicycle frame is steel, perhaps consider also having the rear spacing respaced and the dropouts realigned. That said, you'll need 5.9mm more length on the drive side.

Dishing over approximately 3mm you can PROBABLY accomplish without new spokes--if you start seeing threads exposed on the NDS or you can't tighten the DS spokes any more then you'll probably need to respoke, at least on that side. Because you're likely increasing dish and making the spoke tension between sides on the wheel more unequal, it becomes more critical that this is done to high, even tension, requiring a good wheel builder.