When going across or up into the wind, you aint gonna like having a sail on the downwind ama, unless the amas are nearly as bouyant (big) as the canoe, because the sail will just sink the ama and the whole boat will trip over it and try to flip.
I think you need to find a dissertation on Center of Effort vs Center of Gravity. That's what the whole sailing thing is about: balancing the COE & COG. The amas, on a canoe, are just training wheels to help keep the two (COE & COG) from becoming too unbalanced by leaning over too far. Just like a on a little bicycle.
Matt can surely explain all this stuff better than I can.
I do know that, on a canoe where mast placement, crew (weight) placement and leeboard placement have been properly worked out, the boat can be steered by simply moving the leeboard more or less forward, effecting the center of effort. Also by moving weight fore & aft and, to some extent, by letting the sail in or out. So the real trick is finding those best locations, for mast/sail, crew and leeboard. All the amas will do is maybe help you carry the sail in more wind, or perhaps let you carry more sail from the getgo.
What I think I would do is concentrate on getting the canoe sailing and balanced first. Then you could decide later if you even wanted to fool with amas at all.
Now, if you were only planning to sail straight downwind, you could theoretically stick sails anyplace you wanted, since they'd only be pushing forward with no sideways effort to gum up the works.
That's my take on it, anyway.
