How did you install the servers? Normally UrBackup has a separate identity key for each server, and a client will only talk to servers that match the key(s) in their server_idents.txt file. Only if you copied the server key from one server to another so they both have the same key, or added both server keys to the client file, will the client talk to both servers.
There can be a notification prompt at the client when it sees another server on the network, but if the user is trained not to approve a different backup server then the client will keep using the specific server it first associated with. This prompt only happens once, and defaults to no change.
You can edit or replace the server_idents.txt file to only show the correct server for that client. On Windows it normally lives in C:\Program Files\UrBackup on the client computer.