I am wondering the same thing, as I have 4 - 450+ watt motors, that I may build a B-17 with.
In my nitro days if you put 2 .30 engines on a twin, it would act and preform like it had a .90 motor on it.
People explained it by relating it to the swept area of the propellers. They said the area of props on the 2 - .30 engines, was more equal to the area of a single prop on a .90 (roughly)
I know a couple of twins I built back then, startled the heck out of my when they took off.
I don't as yet know of a calculator for multis.