Fastal, email Andy at SIM and ask him, let us know.
I have the same thing but it differs between twins.
He may say only 2 or 3 pistons fire in neutral but be specific that it USED TO REV higher.
No motor is supposed to be manually rev'd up in neutral, 2 or 4 stroke, car or boat, it's all the same.
What over-ride button? It is either in gear (loaded) or neutral (unloaded).
I did have a problem with one where it would not rev up at all in neutral, would just stall upon opening throttle even slowly, and barely idle, I changed the two LP vacuum fuel pumps per motor and cleaned the O2 sensors (new one on one motor), every gas filter (as usual), fixed it. But one motor does go higher but I'm on the drydock out of water (no backpressure on exhaust). When I kick it into gear however, it is ready to rocket !
Your throttle plates may have to be "synced" to open/close at same time if off a bit (Andy will email a procedure if you don't have manual), check spark on cylinders, perhaps weak coil.