I forgot to mention, it IS possible to wep a single engine, if you select each engine individually and both engines are either running or one is dead from battle damage (with the occasional bug).
I saw Murdr do this on a film one time to do a hammerhead, he has a single CH Prothrottle with engine selection being the 3 buttons in front of the unit.
The poor guys that have dual throttles can't do that, however. Frankly, I think we should be able... my recommendation is a programmable 2nd wep button.