Here is a video. I will try to explain some of it feuters.
When a boat heels the sail area moves to lee, with that the center of effort and the boat gets more weather healm. With biplane rig if you drop the windward sail the center of effort moves to lee. The boat gets more weather helm. With a third mast, a mizzen to balance the center of effort stayes the same. Much as with a ketch if you drop the mainsail the jib and mizzen keeps the center of effort unchanged.
1964 I was a member of the Amateur Yacht Research Society. I did build a proa. To balance the sail area to lateral area I moved the sail sidewise to lee or windward untill equlibrium was reached.
Going to windward with a biplane rig is fine because both mast get free airflow like a biplane airplane.
Sailing down wind the rig is very stable and selfstearing with the sails sheated out more than 90°. With freestanding masts there is no chafe if you keep the lugsail on the lee side of the mast.
Two pictures below.
To be contiued…