change up ur ports on the gameserver and account server on the second server you wish to install any ports not in use really would work........ and make a new database for it in ur mysql install.
however this is a really bad idea considering both servers running on one vps would make both servers vulnerable to the other servers problems ..... not to mention make them run slow from overusing your resources on one server
To be honest from my experience. run 2 servers on 2 different IP's.
Like everyone said if one server is lagging out the other one will. so if someone is speed hacking an it lags out server both servers are going to be effected.
to keep everyone happy just go with 2 IP address. or your going to have unhappy people an there going to leave both servers.
