I've used a 3 speed shifter successfully with a 2 ring crank. I had a 2 speed m665 fd (2 speed) and used a bog standard 3 speed shifter and it worked fine. I was using a 3 ring crank with the granny ring and middle ring so i just used position 1 and 2 on the shifter. The design of front derailleur was such that it couldn't pull past 2 anyway. Depending on how your crank is set up it is feasible. i read that Shimano 9sp and 10sp front shifters both pull 11mm of cable on the first shift and 7mm on the second, so they are interchangeable. so if your smallest ring is in the granny ring situation use 1 and 2 but if smallest ring is in the position normally taken by middle ring, use 2 and 3 and adjust stops. I'm making it sound much more difficult than it actually is , especially if front derailleur is designed for 2 rings anyway.