Put a tee after the primary filter. Probably what you have now. You don't need 2 supply lines to the tank. Your heater uses so little fuel
your engine won't notice. Most diesels pump several times more fuel
than they burn, returning the rest to the tank. If you are having fuel starvation problems you could be sucking air from the heater fuel control, but you'd have heater firing problems. Put a check valve in the heater line.
I have 4 diesels, a boiler and a stove on the main supply line.