You would recommend you to install ISPmanager Pro on the first server and BILLmanager on the second one.
In this case the first server stores users web data, databases and controls all email accounts. The second server with BILLmanager processes all operations on users enrollment. After that you just need to integrate ISPmanager with BILLmanager
http://en.ispdoc.com/index.php/Server_management