I like your multi-engine test stand. it is very innovative, something I never thought about doing myself.
I normally just bench test each engine one at a time and run a few tanks through them to make sure they are working Ok and there aren't any problems. I then mount them in the plane and fly them on the rich side some and take it easy getting used to the plane, et cetera. Then as I fly the plane i lean out the engines as needed.

I have found that over the years of flying twins etc. that the engines do not have to be synchronized all that much, they can be off from each other. Normally I adjust the strong engines to match the weakest engine if possible, but even 500 or more RPms off between engines doesn't seem to do much to the plane's handling in the air. It is a little aileron trim and that is about it.