Ok I havr twin 454 mag mpi if I turn either ign switch on it powers up both engines ( ECM,fuel pump and mad ign but to start engines they will only stat with right and left ign switch. If I leave right battery switch on and shut left one off then the right key switch powers only the right motor and vise versa for left it I turn both battery switchs on then either switch powers up both engines batteries are seperated and ground and power cables go from each battery to there own engine please help thanks

How many batteries are on the boat?
Are you using an isolator or something else to charge more then one bat?
What is used to combine batteries when needed, a switch or relay?
How long have you had the boat?

One battery for each engine each engine charges it's own battery and they have seperate battery cut off switchs just got the boat in March of this year

I understand how one key can turn both motors ON, and not be able to start both, even though it shouldn't be able to do either.

The PO had something installed and when removing or rewiring something, there was something done.

Look at the Key switches and see if there is a jumper or other connection to the purple wire somewhere. With the key OFF, use an ohm meter and check between the purple wire on each key run terminal (purple wires). There should be no continuity between the two. If there is, start looking at wires behind the helm for a jumper between gauges

If there is only 1 voltmeter look for a switch that allows one motor to be looked at one time. If there are 2 voltmeters, see if they were swapped in some way in wiring.

I no isolator is installed, look for something that may have been installed before

maybe this diagram will assist you, each wiring harness should be isolated from the other one,

I will look tonight do you think that would cause ign problems ran boat 2 days no problem end of second day lost both motors with in a 150 feet of each other no spark got it home did some probing both motors fired up and ran

Sure, there is something that was done and what ever it was, it should not have been done

Thank you I will be sure to post my findings

sounds like grounding issues,

I think I am convinced it has something to do with the purple wire behind dash