I have an Avid on the back of our CoMo with Campy Ergo levers. I also run the inline travel agent. The Travel Agent and "helper spring" are two different items. Some have placed a small spring over the cable where it attaches to the "arm" of the disc brake caliper. I've tried this and hated it. I was fortunate to find a longer spring adjustment screw for the Avid Mechanical. The first generation disc brake caliper uses a small fine threaded screw to adjust spring tension on the brake - the helper springs purpose is to increase this so that lever feel is better with such a long brake cable. Some company was giving this away at the Northwest Tandem Rally in Bend Oregon in 05 - actually at the Arizona Tandem table. I use the travel agent to reduce brake lever travel. The system works great.
A friend just picked up a new DiVinci with Campy Ergo Levers and the new 2nd generation Avid Mechanical discs front and rear. The disc caliper design eliminates the screw for spring tension - and i'm not sure how it's adjusted now. He does not have a travel agent inline and his lever travel is about the same as mine with similiar "tension". I'm going to be looking into the 2nd generation caliper so I can eliminate the inline travel agent as it's a hassle to set up when installing a new brake cable.