My rule of thumb is to have them separate. Just in case one fails you don't loose the hole system. But that is just me. I should draw up how my fuel system is set up. It will blow your mind. You can start the truck but it will only run tell the float bole is empty. To start the truck and keep it running you must first turn on the ignition SW then press and hold the fuel prime button tell it's up to pressure and the the engine is running. Then the gage keeps it running. If the pressure drops below or above the adjustable set point then the relay opens and the fuel stops. But if the gage fails then I can bypass it completely and just force it to pump fuel. It takes 5 relays for the hole system to run and not have the warning system activate when I just leave on the gages to check maximum recall points. Only the fuel gage can shut down the engine and trigger a warning, the others just set off warning lights and buzzers. It just kind of took on a life of it's own. But it has passed all the bench testing.