If using the same SSID the client will decide whether it uses the 2.4 or 5 GHz band. There are bad clients there, but most laptops / phones will prefer the client with the best signal strength. I suspect you played with the transmission power but that's just an educated guess. It's a general misconception that a high transmission power equals to better coverage or throughput speeds. It will hurt performance instead, especially when we talk about the 2.4 GHz band.
So if your client always prefer 2.4 over 5 that means you haven't optimized your WiFi settings correctly. I always recommend a high transmission power for 5 GHz and a low to medium transmission power for 2.4 GHz. What works best is dependent on your situation. See this post I posted some weeks ago.
// I accidentally quoted Juppin but this reply was for OP. Anyway, proper optimization preceeds using 802.11v imho.