Redirect using GET variable
Can we setup the dual URL testing so the original URL (http://www.xyz.com/homepage.php) redirects to the same page with additional GET variable (http://www.xyz.com/homepage.php?showNewHeader=true
This way we don't have to create 2 pages since we just want to test the header change.
Not sure what you mean by additional GET variable. But, yes you can redirect with the only change being a query parameter added to the URL. See this article in developer docs on advanced redirects.