I have the factory flush system from Fountain. One inlet serves both engines. There are levers and faucets that control which engine gets flushed.
I keep the instruction manual page (copied) to refer to so I don't screw things up. I only put in the key to start the engine I plan on flushing.
I also wait until water is flowing out of the drive and I hop out after the engine fires to ensure water is flowing out of the exhaust pipes.
US1 Fountain will understand... I do like the system as after I finish flushing an engine I can adjust the faucet to back-flush the drive.