If not I suppose you could send GET to two iframes in the formhandler page - set the src values to the siteXurls and attach the querystring to each. It's not elegant and I haven't tried it but I assume it should work.
I've never tried it before, but can't this be done easily with JS? Use AJAX to submit the form once, then either use a callback function to submit again, or let the form continue processing and go to the action page?
To be fair though I suppose using PHP is better in case the user has JS disabled or is using a non-JS browser.
by EvolutionFallen: had a change of heart ;)
If using ajax/js - you must use a cross-domain solution. I'm not sure that a vanilla jQuery ajax routine will work with remote sites.