An an example, the Swift (low wing) is just "both". There are a lot of
examples of low wing planes with "both".

In my Mooney, if I ever run out of gas, I'll just switch tanks. Thank
God there isn't a "both"!



There is a significant difference here between low wing, and high wing
designs. If you have a low wing aircraft, and have a 'both' selector, if one
goes empty, air is sucked into the system by the fuel pump, and fuel
delivery stops. Hence low wing aircraft with a 'both' position, have to have
a central 'sump' from which the fuel is drawn. On high wing aircraft, where
the fuel is delivered by gravity, if one tank goes dry in the both position,
fuel will still feed from the other. Hence 'both', is relatively unusual on
low wing designs, but more common on high wing planes. This is why your RV6,
doesn't have a 'both'.

Best Wishes