It's difficult to interpret what you require, so let's use some key words.
The main web page is called the parent window, and the popup page will be the child window.
What is it that you want? Is it that when someone clicks a link in the parent window, that that link is loaded in the child window replacing what was there?
Because if so, that's easily achieved by using the same name for the child window, and for each of the window.open commands to load a link in to that child window.
Or, is it that you want the link on the parent window to open up in the child window, and for the parent window link to be replaced by a different type of link?