There is a pretty lengthy thread about the TRP Spyer (which was sort of hijacked by the TRP HY-RD) calipers on the tandem forum (sorry, screen reading software doesn't play well with the site and I can't post the link, but I thin the thread is "TRP Spyer Equals Bee's Knees). I think most calipers use one "stationary" pad and one moving pad which flexes the disc into the other while [again I think] the TRPs move both pads (making adjustments and set-up easier). The HY-RD (mechanical to hydraulic) versions also adjust to pad wear.

The OP's issue really sounds like a lack of cable pull. OP mentioned "short pull" Campys, even with the right Avid road calipers could this be an [the] issue?