wrote ...
Two low-wing tanks require left/right control to avoid
sucking air from an empty tank. No Both position, or you'll
have trouble, especially if they have separate vents and the
tank pressures aren't exactly equal.

Great thread! I've always believed the above also, but the certified Beech
Skipper has low wing tanks with individual (45 degree cut tube) venting and
a single Both/Off selector. Each tank feeds thru an inline check valve into
a common header tube below the cabin floor, then single outlet to On/Off
valve then to gascolator. The design does try to keep common tank pressure
via a tube interconnecting the top of each individual vent tube. But unequal
feeding is one of the most common complaints.