thats because the 2 guages dont actually show aux/main, they show total internal/selected tank. would be nice if they showed the separate tanks as IRL, although the guages for the wing tanks on the N are tiny and would be almost unreadable.
the N's individual wing tanks shouldnt be selectable either really, the options should be: main/aux/internal wings/left external/right external/centre external. interestingly the N manual says the normal order was 15mins main tank, alternate external tanks every 30mins til empty, aux tank down to 25galls, 10mins main, internal wing tanks til drained, then main again. so the aux was burnt off before the wing tanks, unlike AH where the wing tanks are burnt off first (which helps with rollrate.)