A quick javascript hack would be to intercept the form submission and submit the form twice, changing the form action so that the same data was sent once to each domain. You would have to set the target of the form to a hidden IFRAME for the submission whose output you didn't want to display. Attach the javascript to the onsubmit event but add a control to stop the form submitting endlessly (since submitting a form from javascript will trigger the onsubmit event).

The problem with the scenario is that you are dealing with the POSTed data (in the form of a picture). If your issue was limited to simple arguments that could be submitted using a GET then you could just redirect the user from one script to another.