the two orbi routers would not work as a single wifi system , you would still have 2 transmissions and they would interfere with each other. So you can’t have a single ssid across the 2 routers in the “system”.
What you could do is take the satellites from the new system you bought and add that to your existing system. That would give you a router and 2 satellites. Of correctly placed they should cover about 7500 sq feet (marketing numbers to get maybe some resemblance of a signal all over the 7500sq feet). Maybe a more realistic number would be to use approx 3/4 of the advertised coverage as a better model for “good” coverage throughout the property with various types of walls/interference/openings. So a 3 unit rbk50 should have no issues with a 5000 sq foot home.
