Hi Bill, I can't remember which books (and there are many references to this effect in quite a few books) but from memory, the most comprehensive explanations are from the 70's to 80's for adaptations of "lee" boards as well as twin centre boards (and rudders) on shallow draft sailing mono hulls and sailing barges, sailing and working on the lakes and canals of Holland and Germany in particular, but also for pleasure craft in the Florida region. It would seem that to greatly reduce drag (and still have a similar efficiency to a more conventional single board) from having both boards working at the same time, they had to be set up "cantered outwards and toed in so that they "pointed" to a point approximately one third of the length of the hull, forward of the bow, and the rudders set up to point to that same point. This is a very "approximate" description as I am relying on memories that date back some 30 odd years. The idea to have twin boards recieved some popularity for a short time for "pleasure boats" as it had some obvious advantages over deeper foils, ie to be able to sail and cruise in shallower waters (as the actual depth of the cantered boards were not much deeper than the keel line of the boat, but due to the angle of the board, there was still a respectable area of board in effect), to be able to "beach your boat in tidal harbours and when the hull was sitting on the mud the boat would still be in an upright attitude instead of laying on its side (popular in a lot of European harbours), and for working, sailing barges it was, of course essential.
The adeptation for these "twin" boards on a mono is different from that of a multi hull only in the number of hulls, but the principles for both are the same.
Darryl