There's no way to have a client be joined to two servers simultaneously, so the simple case is out.
You can switch a client from one server to another by running
C:\Program Files\Windows Home Server\Discovery.exe and selecting a different server (among those servers detected on the local network) in the dropdown. But that won't get it to back up automatically to two servers, and I don't think you can script the server choice. So I would have to say that the answer is no, you can only back up to a single server without manual intervention.
I'm not on the WHS team, I just post a lot. :)