you'd have to award equal points for race standings in both classes and two winners. or, have classes switch after one race and have the overall point leader(s) after both races the overall winner.
EDIT: the miata sounds awful!
Two winners doesn't work for the same reason that having two different races wouldn't work - much would depend on how competitive your 'group' is.
The Miata sounds pretty raw with exhaust upgrades! Not the most pleasing noise, but it certainly sounds like a race-prepped little Miata.