I am currently reading "Sail Performance" by C. A. Marchaj, and he talks about the crab claw sail being the most efficient sail. This sail configuration also uses two masts.
I also found a site which summarises Marchaj's findings and has a picture of a catamaran with this sail config.
http://www.schachtdesign.com/proafile/volume_3/options_crab_claw.htmlit makes interesting reading