How can i attach two forms with one form?

How can i attach two forms with one form?

Dear Members,
I am create a form which value are depend on other two form then how can i create this type of relationship. like one is type of login form and another two is type of signup pages. now how can i create both (signup) pages relationship with only login page. means when user type any information in the login page then it is fetched or verify data from  both two  pages.

Thanks in advance

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