Micheal, I think you will find the combined power of twin engines are only equivalent to 75% of a single outboard. So twin 40s are about 60 hp and twin 60s are about 90 hp. BIBOA used to use this rule in the early days to try and match boats into classes for racing.
Your BWM orginally had 2 x 40 mariners fitted which were a disaster as the boat only reached 18 - 20 knots with divers. The REME then fitted a cheap Johnson 150 V6 which subsequently blew up.
Look carefully at the engine weights before going down the twin engine route, you could upset the balance of the boat with big twins. Early BWMs had twin 0.5 inch longitudinal marine ply stringers glassed in with a 0.5 or 0.75 inch deck glassed down on top and not alot else.
You really need to talk to Paul Lemmer who used to run BWM. His e mail is: email@example.com
Think he does brokerage and consultancy now. Pete