It seems to me that the power of the two engines combined should be a little more than the power required if there was engine only, to allow for continued flight in the case of an engine failure.

I'm guessing, but I believe the two engines should total something like 120 percent of the power of a single engine. (Say, two .46 engines instead of one .75-size.)

I don't know that there is any "official" standard. Hope this helps some, anyway.