Figured it out. As close as I can tell, for my set-up the front ring needs to be 3-4mm outboard of the rear ring for the belt to track properly. Needed 3mm of spacers to get the rear ring off the chainstay, and 11mm of spacers up front. BBs are both DA 118.5s, so I guess the captain timing crank/spider is sitting a few mm inboard of the stoker timing crank/spider. Took awhile fiddling with spacers/bolts/nuts to come up with the magic combination.