The kill switch I have has 2 wires on it,can I just install each hot wire from each distributor on the kill switch?

I can’t speak to your exact question, but since the DUI requires at least a 12g wire for power I ran a wire directly from the battery through a relay to the ignition using the factory purple wire as a trigger. You could run the kill switch wires in front of the relay should you go this route.

All the boats I've seen had a kill switch for each engine. The kill switches are wired in series with the power to the ignition with no intermediary relays. A relay can stick and therefore doesn't meet USCG standards. The way many after market ignitions work is they get power direct from the battery and run a voltage sense wire to the key switch through the kill switch to turn the system on and off. If the DUI system you are running is USCG approved this is sufficient. This typically is very low current. This is the way the Marine version of the MSD and Daytona Sensors system works.

Relay itself or relay for ignition ?
Some OE engines have fuel pump and starter relays (amongst others) in the engine compartment. Granted the relays are Coast Guard Approved.