Best is a simple, self-made manifold out of 3/4" schedule 40 PVC parts
available in every hardware
store. For 2 tanks plus 1 common plus 1 spare (always add a spare!) you need two T's, 4 valves, 4 adapters to 3/4" thread and a foot or so of pipe. Use PVC or nylon threaded parts
: 3 x 3/4" thread-to-hose for your size hoses and 1 x 3/4" plug
for on the spare.
Use the valved that use cement for connecting to pipe, not the threaded ones.
It will outlast anything else aboard and is very quick and cheap