Generally speaking you need a minimum of around 50hp/ton to get a boat onto the plane so it's going to depend on what you want to do with it. Loaded with divers and their gear It's going to struggle.
With just a couple of adults you're probably looking at the right side of 20 knots.
If your budget's tight do you want to be feeding and maintaining two engines? (probably going to have to re-prop them to get the benefit.)
I'd try it with one of the 40hp & if that's not hacking it, sell them both and get something a bit bigger.......I'd have some reservations about the 26yr old Suzuki that's lain for five years, remember outboards seldom get the opportunity to wear out, they die of neglect and not being used