Should you have a ground wire run between both motors grounding both motors together?

The battery grounds are doing the same thing. another wouldn't hurt, but probably wouldn't help either.

A common ground between engines is a must. It should be the same size as your battery cable. I think that it is a CG regulation to do so. It will prevent smoking a small ground wire if you loose the main ground while cranking your engine.

Found it!

http://law.justia.com/cfr/title33/33...4.9.222.5.html

If you have "1-Both-2" battery switches that allow you to power either engine with either battery, you must have a heavy-gauge ground connection between the engines to carry the starter current. That being said, some boats use a grounding/bonding plate where all the battery and engine grounds come together. That will serve the same purpose.

Interesting

There's a law for this?

Apparently it is a coast guard reg. Also it is common sense. As I stated in the above post you can melt a wire and potentially start a fire if a main ground is lost during cranking. I own an auto repair shop and have replaced more than one clutch cable because of the above scenario.