I'll make a wild guess and say that the 'humble Chipmunk' wasn't designed and certified under the FARs (at least not the current FARs), but there may in any event be components in the fuel system that mitigate the risk of air being drawn into the fuel system due to one one tank emptying before the other.
What happens if a Chipmunk has one tank full and the other one empty, and is started and flown with the "simple ON/OFF selector" in the "ON" position?