I was chatting with the owner of R+E Cycles this Spring as my wife and I were getting fitted for a new tandem. The conversation wandered into the realm of forks and Dan told us a little story about a sales rep. who had put a non-tandem carbon fork onto his tandem. Dan told him he was crazy, but he insisted on doing it. The next time that rep. came into the shop, he had several new scars on his face. The fork had failed and parts of it had gone through his face. I would guess the hospital bills were larger than the cost of a proper fork would have been.
I don't believe I would ever put a fork on a tandem that isn't designed for tandems. If I want to carry less weight, I just get rid of the excess carbon on the engine. That's the low-hanging fruit (and nuts, and cakes and...)