How to preserve loging on iframe?

Question

I have two tab in my webpage in one tab i have some textbox, dropdownlist and button and in another tab i have iframe by which i am calling another site in my page.
but when i am login to iframe site and going back to tab one and filling some data in textbox and pressing button for save data.after that i see that i have log out from second tab website
how can i preserve that login.

can any one help me soon?