You can either create oracle instance manually or by using
Oracle Database configuration assistance.. ODCA is a GUI very easy to use. You can also customize your database in ODCA GUI.
Which version of oracle database are you working with or want to create? What is your operating system? Please add more information.....
Whats your reasoning behind running 2 instances for 1 database. It can be done using OPS if you are looking for scalability or failover capabilities. If those are not your requirements, then its not recommended.
BTW, OPS is a completely different animal to tune and to manage.
Please post details about what you are trying to accomplish so that we can help you more.
I am really sorry i could not answer your mail in my personal mail id.I am always of the firm view that if we put it in this forum it may be of use to all.
As i told in mail many may give a better solution then what i say and even if i make a mistake some one can correct my idea too.Thats was the reason i told you to post it in the forum.
Please follow this oracle document which tells how to set up up multiple instances.
The first link explains you very diagramatically on the topic of your interest.