I'm not an expert in this field but willing to help
I think you can add the second server without problems. So you configure the server and add it to the first. Make sure your second server has access to the db of course! So change your my.cnf and restart mysql.
After you've added your new server you can setup new sites on the new server. I just tried to move databases from one to another and that's not possible. So don't expect a fully automated transfer.
I think the solution is to add the server, create new sites, transfer the data (db + web) and then change your dns settings.
If I'm mistaken and there are better solutions, I'm interested as well