If you are not using any Xfi Pods, you can try separating the 2.4 and the 5.0 bands and give them two different broadcast names / SSID's;
Is this the result of a diagnostic scan or just which connection the gateway has decided for your particular device? Out of the box my XB7 sets a single SSID and password but both bands show up under scan. As was mentioned above, there is the option of making the bands discrete providing you're not using any pods. The xFi UI leaves some things to be desired but the option is in there.