How many rpm difference is acceptable between two engines when they are running at any speed from cruise up to WOT. With modern fuel injection I know it is possible to have the engines synchronized to each other very closely if the throttle bodies are fly by wire, but what about EFI engines that still have a cable operated throttle body. Does Mercury or any other manufacturer automatically sync their engines above a certain rpm?

It's kinda of important for stress relief. You may have to adjust/replace throttle cables but once you get it is a sound. The sound is more telling than any tach. There is a harmony there and you will know. Even if sticks ain't perfect.

And what a sweet sound it is...............

I have the tachs and a syncro gauge.....but it's the sound!!!! You'll know it!!!!
On the remodel the Syncro gauge is leaving the panel. Unnecessary space!

^^^ Yes indeed..

With drive trim.. cable adjust... prop variance and even boat lean that can effect engine twin synch. I always eyeball the tach for a quick look as i twist the sticks to harmonize it .
My buddies Cruiser yacht has a actual synchronizer that runs off the coils. Haven't got a grip on that one.

How sweet that sound is.