I have the latest Airport Extreme and have two networks SSIDS. One for 2.4ghz and one for 5ghz.
Does anyone know if the iPad Air can still take advantage of MIMO in this configuration?
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.