Carbon bikes aren't that fragile! Sure they can brake but so can a Ti bike. Destroying a Ti bike in a race can be just as expensive as a carbon bike. Have you priced Ti tandems lately? Any performance tandem that they get will be expensive.
Funny, I've broken a Ti frame but never a carbon one and I've been on carbon, almost exclusively, for the last 4 years or so. In fact I t-boned a van (on my single bike then) at 40mph during a race in 06 and the only damage besides me was a broken Zipp wheel, a broken carbon crank arm and a broken shoe. I was able to get back on the bike and finish the race on that bike after I got out of the hospital. I did my first (ultra) race on my Calfee in June, have another race in three weeks and will be doing a bunch of TT's and probably some short road races next year. I used to race/ride a Santana tandem and I don't regret the switch one second.