Instead of parelleling electrical lift pumps, better to install a 'day
tank' that is constantly supplied by freshly filtered fuel oil but with
a lock-out cock (to an inverted dip tube in the day tank) so that the
day tank always keeps a full load of fuel. Then if you have electrical
failure or filter blockage you will be able to use the 3 or 4 gallons
in the day tank to keep moving and make repairs, etc. If worse
becomes worse, you can always siphon fuel from the main tank(s) and
manually load a gravity feed 'day tank'.

FWIW ... A Walbro IMHO is NOT a constant duty delivery pump. A better
solution for that application is simply an automotive 12v impulse fuel
pump. The Walbro is an intermittent transfer or recirculation pump;
but, can be so piped that if the engine mounted diaphragm lift pump AND
the inline priming pump BOTH fail then the recirculation pump can be
used to either pressurize the system or deliver to the 'day tank'.