You should use the same SSID for 2.4GHz and 5GHz. You should also use the same SSID for all APs if they support roaming. Unifi does, for example. If you just get two random independent wifi routers, they won't and you might stay connected to the wrong antenna for a while.
https://community.ui.com/questions/Wireless-LAN-Roaming-FAQ/...