Is there way to change both windows in testpage.htm (using the Ctrl - N method)? And in which browsers will this work?
If you only have control over page1.html, and cannot alter the code in testpage.html and blank.html, then I would say that there is no easy way of doing this at all - But, I look forward to being corrected! :)
Once a user opens a new page, independent of any of your script, then you have little to no control over that page.