You know that MIMO isn't to span two different band frequencies right?
MIMO is multiple antennas working to transmit and receive (usually a 2x2 or a 3x3). Your Airport Extreme supports upto 3x3 because it has 3 antennas. Whether you are transmitting on multiple bands has nothing to do with it.
And yes, from everything I have read the iPad Air has 2 antennas so therefore it can utilize MIMO and will do so EITHER on your 2.4ghz or 5ghz bands.