Waldo:

Given that Cannondale's current street tandem (low cost version) uses Cannondale's equivalent to the Avid SD-7; you are probably fine... Just remember the Travel Agents.

An alternative are Shimano R550s (or equivlents) which weight less and don't need the Travel Agents.

But why not go to calipers on the front on a new Carbon fork? ( I am also trying to get this through Parliament and get the Governor General's signature - Canadian equivalent Eh!...but have a Fox F100X on the wishlist for my ATB frame that is in conflict and higher priority)

So while I think you are NUTS to be replacing the rear disc, as you have pointed out it is YOUR money.

Carbon bars are nice on Aluminum tandems as they kill vibration in addition to carving grams off.

To answer a previous question - Stealth Cycling sells a carbon stoker stem...not sure who makes it for them...

Don
(Avoiding the tubular nonsense )