In a combo that has internal fuel tank would be a real PITA to disconnect fuel hose from OB entrance, unless has a quick release connector like the ones used on portable tanks. According to my personal experience with 2 stroke 3 cylinder, 3 carb OB's, not an issue, can connect a spare female connector on OB's fuel entrance so internal central pin of male connector remains pressed and open, that way while OB is on while flushing at idle, will consume most of bowl's fuel on each carb till OB drops dead.
This procedure is excellent if you're not hooked on constant boating, don't know when your next outing will be, one week, one month, several months ? carb will remain immaculate interior clean and gum, varnish free. I really laugh when boaters stops 2 strokes multicarbed OB's for 2- 3 + months and suffers starting OB at the marina. I have issues with E fuels, are evil on thin fuel pump diaphragms.
Happy Boating