below is the value assigned to the send-to variable:-

" "

a value between the speech mark proves that the variable $_SESSION is working at this stage

From these reults it would seem that the session is working however if we now revisit testpage1 now that the wfb script on this page has been run we will see that there is no longer a session value available.
If we return to the set session page to reset the session you can refresh testpage1 as often as you like and the variable remains working until testpage2 is loaded resulting in the wfb script being run. You will also see that although the session variable shows above, if you refresh this page now that the form has loaded you will see that once again the session has been broken.