fandango

Hoping someone can help as I am out of ideas. Twin 575 SCI's just rebuilt at Innovation. Both motors run good with no issues. Problem is when starting. Start one and let idle. When go to start other, sometimes it will kill running motor. Does not matter which one I start first. Does not matter what the battery position is on. I am having a hard time isolating batteries to individual motor. Even when disconnect large ground from between motors, and have battery switch set on 1 for port and 2 for starboard, when go to start non running motor, I can see voltage drop on running motor???? Change batteries, same issue. Both alternators are putting out 14 vt. Checked all grounds. Both starters turn over quickly and each individual motor will start immediately. I am sure I am missing something. Ideas?

Thanks in advance.

Ed