Depending on your model of wireless router, you might have the option to have the same network name broadcasted on both bands so that it appears as a single network. Under this configuration, clients will automatically choose which of the two bands to use. However (and this is a big however), many client devices aren't as intelligent as you might expect them to be and can frequently make a non-optimal choice.
The advantage of broadcasting two separate SSIDs is that you can manually choose which band to use. 2.4 GHz is slower and more prone to interference due to its limited number of non-overlapping channel options, but lower frequencies are generally better at penetrating walls. 5 GHz is faster and will likely see less interference, but may not be supported in some devices with older or low-cost WiFi chips.
Generally speaking, you should prefer 5 GHz unless you are using a device that doesn't support it or if you are in a spot where the 5 GHz signal is very weak. You can also try running a speed test on each band for each device to see which band gives you better performance on a per-device basis.