Twin outboard issue

[Loyal to none]

I have twin 225's on our boat, this weekend after running both motors before leaving home, both running fine, unloaded boat, fired starboard motor right up, but when I tried port it would spin and try to fire. After checking fuel, changing plugs, removing the check valve in fuel we found that when the starboard engine key was turned to start it the port engine also try's to start. We have replaced the entire key switch unit. I have checked the wiring harnesses on both engines and cannot find anything that is crossed as they have different harnesses. If the power switch is off it will not do it. only thing they have is a common ground which I have cleaned.
Anyone have any thoughts