Will performance on my AirPort Extreme be hurt pushing AirPlay content over 2.4Ghz and the rest on 5Ghz?
But if you separate the two by SSID, giving you a dedicated 5Ghz and dedicated 2.4Ghz band, does it still transmit on both of those simultaneously to separate devices? 


Yes (non-issue if the Airport Express units connect on 5GHz).


  ...so I'm just curious if there's anyone familiar with this or if it's even necessary.


I've had various Airport routers since 2007 and I've always separated the 5GHz channel from the 2.4GHz both at my parents house and in my flat downtown. If you can obtain good coverage with 5GHz then that's preferable because no nearby networks, cordless phones or microwaves will interfere with your music playback.

The main problem is not network congestion, it's interference.