How can i merge forms in custom entities like in opportunity form in crm 2013

Hi,

I want to merge two forms in custom entity like in opportunity form in CRM 2013 . When I add stackholders in opportunity form It opens within the opportunity form. Same as I want to do with two custom entities for example : new_opportunity,new_stackholder. Please guide me how can I do that?

here is opportunity form example

When i create stackholder from opportunity from then contact form displays with in the opportunity form (within one window) same as I want to do that with custom entities forms. How is it possible

Contact form is displayed with opportunity form