Hi,
for what it's worth, I've posted this on the wingsail forum I've now started up, under the topic "one mast or two?"
"On a monohull, the break commonly comes at an overall boat length of around 10 metres. Below this length, it is difficult to fit a schooner or ketch junk rig into the space available. Above this length, a single sail of appropriate size is getting to be too heavy to handle - a sail of 600 sq ft/ 55 sq m is about the practical limit. However, a single sail is usually thought to be more efficient than a two-sail rig, and so for performance, is preferred. A two sail rig is preferred for ocean cruising, for its greater choice of reefing and trimming options.
On a catamaran, there is no choice. Two equal rigs are made and fitted, one in each hull. There are several junkrigged catamarans treated this way. And remember "Team Phillips"? Nice rig, shame about the hulls. In New Zealand, there is a successful cat called "Cool Change", with a wingmast in each hull.
A trimaran is treated in the same way as a monohull.
A proa? Well, that's breaking fresh ground. My hunch is that a single sail will be easy enough to deal with, but two sails on the lee hull might be a step too far - I can't see how they can be sheeted. For a proa that is too big for a single sail, I would want to think about a "schooner" rig, with a large sail on the lee hull, a small sail on the windward hull. The side-by-side rig is successful on catamarans, and I see no reason why it should not be successful on proas.
David.