It sounds like the systems were set up as two non-connected systems, which they shouldn’t have been, unless they are running two separate operating systems for Sonos (S1 and S2).
You would need to factory reset one of the two systems, and the add those speakers back to the remaining system, using the ‘ add a device ‘ function in the controller, so everything is in one ‘house hold/system’ and you could then group them.
Note that a factory reset will erase any stored data, from associated streamers, playlists, and TruePlay features, it’s a very invasive thing to do. It may affect which system you choose to erase so that you can add it back to the other.
Also, this is all moot if those systems are running S1 and S2….