I'd guess that one reason it doesn't work very well is that when you have twin 50's they are propped as "half of a pair." If you were purposely using only one engine, I believe it would have a different prop (propped as a single). (This has to do with attaining rated WOT RPM so engines aren't lugging.)
Not that a 22 with a single 50 would be a speed demon, but I bet it would plane a lot better than one with twin 50's only running on one (presuming things are propped correctly).