Hi @sigedil671, welcome to the community.

If you only turn on one half of a stereo pair, then it’s unlikely that your speakers would show up as a playable room at all. If it did show up, then it would indeed play “full stereo” as if it wasn’t in a stereo pair. 

I recommend manually separating your stereo pair if you plan on only using one of them at a time.

I have tested if I switch on only the left speaker, it works unless the right is switched off. It seems stereo.

If I switch on only the right speaker, it doesn't work. In the app appears not configured.

Yeah, this is what I meant when I said it may not show up at all. 

When you stereo pair two Sonos speakers, one becomes the “Group Coordinator” (GC) and the other the “Group Member” (GM). If you power off the GC that basically cuts the connection for that room. The GM is looking for the GC to share its connection and music information, so without it the player doesn’t know what to do. This is why you should separate your players if you plan on only playing them one at a time. 

In your case, it seems like your left Sonos Five is the GC, which explains why the room doesn’t show up if you power that one off.