Well I dont think it will create problem. Right now I am using two instances in the same server, started running simultaneously.
Only issue is, u must have sufficient RAM size, processor capacity, proper disk I/O load shed and proper Init parameter tuning. Otherwise the server will suffer performance issues.
What is the server capacity and its details, oracle version and the database sizes? Max. user sessions expected etc., etc.,
There is nothing Impossible.
Even Impossible says
I M POSSIBLE