Your AirPort Express is already providing two separate 2.4 GHz and 5 GHz bands, but each band is using the same wireless network name.
This is the default setup for the AirPort Express, which is recommended for most users. The theory here is that devices will automatically connect to the best quality signal based on their capabilities and distance in relation to the AirPort Express.
It is possible to assign a different name to the 5 GHz band, and then "point" devices at that network to connect. Some users swear by this option.....(I am not one of them).... but you might want to give it a try to see how it works for you.
Open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the AirPort Express
Click Edit in the smaller window that appears
Click the Wireless tab at the top of the next window
Click Wireless Options near the bottom of the next window
Enter a check mark next to 5 GHz Name.....which will automatically add "5 GHz" to the network name....so you can identify it
Click Save, then click Update and wait a full minute for the Express to restart
Now you will need to "point" your 5 GHz capable devices at the 5 GHz network name. 2.4 GHz devices will connect to your "other" network name.