First get rid of your IF/ELSEIF statement. When stepping through the code in debug you will see that it will only execute the code in the IF portion, and will never execute the code in the ELSE IF portion.
If that doesn't work, try naming the windows differently. Currently you are naming them both "PrintMe". Change one of them to "PrintMe2" or something and see if that works.
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Okay...now I'm thinking it's because you are creating two OnClick events. Can you put the javascript for both window.open calls in a method and then just call that method in the OnClick event?
Here is a link with to a sample that I found:
http://www.java2s.com/Code/JavaScript/Window-Browser/Openmultiplewindowsatoneclick.htm[
^]
And here are some other google results that may help:
https://www.google.com/webhp?sourceid=chrome-instant&rlz=1C1EODB_enUS581US581&ion=1&espv=2&ie=UTF-8#q=javascript+window.open+multiple+windows[
^]