It is an extremely simple fix. The kill switches break the ignition wire after the switches. Take out the two kill switches and splice the wires together to make both sets of wires to run to one switch. That way when the switch is off position both engines will not start and if you are running, both engines will shut down. Simple.
Here is a rough diagram on what you need to do.
http://www.hotboatpics.com/pics/data...598wiring1.jpg