i think they could be.. the 2011 opel astra has this combination, i can't find a drag figure for it so it may not be all that great, but considdering ground clearance and approach angles airdams can never go as low as you'd want them, but being closer to the tires wheel dams can.
the combination i think is best and what a lot of low Cd cars run is a central undertray, lower side airdams in front of the wheels and suspention and small wheel dams as pitures