6 mm tubing, tanks in parallel, and double vent.

Make a test as follows : run the motor at full throttle for about 2-3 minutes and observe eventual collapsing. Only at sudden idle RPM you will notice the tendency to flame-out because of underpressure built up in the tanks vs the too low voltage of the fuel pump.

Note that the more empty the tanks are, the easier the fuel flow will be, thus less tendency to flame out or to collapse the upfront tank(s).

Nicolas.