Will performance on the AirPort Extreme be hurt pushing AirPlay content over 2.4Ghz and the rest on 5Ghz

I already know individual answers to this but stick with me 🙂 So, the latest AirPort Extreme (802.11ac) says it can do simultaneous dual-band, which is fine when you're using both frequencies together. 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? To give you an idea of what I'm trying to do, I want to hook up AirPort Express units around my house to provide AirPlay to assorted speakers in the Kitchen, Dining Room, etc and was thinking, in theory I can have them all operate on say, the 2.4Ghz band while the rest of the house is on 5Ghz, and it should alleviate some network congestion when streaming to multiple units at one time. In my mind, it's like Sonos but a bit cheaper since I have most of the hardware already. This may be completely pointless, so I'm just curious if there's anyone familiar with this or if it's even necessary. I've had multiple people tell me to use the 5Ghz band exclusively since my house is relatively small and I can hit it from almost anywhere. Is my logic misguided?