My wife and I do about a half dozen tandem TT's a summer. I actually put together a tandem specifically for TT's with aero bars, bullhorns, bar end shifters and the lot. It's a Cannondale (same frame we normally ride). I put a Bontrager carbon road tandem fork that takes a long reach road caliper on it to lighten it and lower it. In the rear I still use a 6" rotor disc brake. For the captain's post I use a Profile fast forward and then use a standard post and bullhorn bars for the stoker.

One place I don't dare scrimp is on wheels though. Light and aero are great but flat out in the aero bars the bike can be a handful to control. You also go really fast. For the TT that we do (rolling 9.75 miles) we are averaging about 29mph and are doing almost 40mph at one point. No way I dare risk an issue with a carbon wheel (or most any other part) that wasn't designed for a tandem and the forces there of. My worst fear is losing a front wheel at speed on a tandem. It would be catastrophic and the captain would be lucky to survive, based on the low speed tandem MTB crash I had once where the stoker ends up driving the captain into the ground.

Mike