we have lookup relationship from custom object to the opportunity and with the help of visual force page or trigger I want to show both objects records in each other’s related list.

What kind of relationship will it be?

• Many-to-Many between Opportunity and Cusotm Object ( n:n )
• One to Many between Opportunity and Custom Object ( 1:n )

You can achieve both things even without a visualforce page. Do you have a specific use case for the VF page?