@Caullla 

It is not possible to separate Deco Main Network to two SSIDs, one for 2.4GHz and another for 5GHz.

For Deco X50, you can configure preferred band connection for each device in Deco app. The process is described in the following document, Part 2: How to select a preferred signal source on the Deco App

If you don't see that feature in your Deco app, check this forum topic: Deco X50(V1) 1.1.0