The currently chosen best answer is too fuzzy to be reliable.
This feels to me like a fairly safe way to do it:
(Javascript: using jQuery to write it simpler)
$('#form1').submit(doubleSubmit);
function doubleSubmit(e1) {
e1.preventDefault();
e1.stopPropagation();
var post_form1 = $.post($(this).action, $(this).serialize());
post_form1.done(function(result) {
// would be nice to show some feedback about the first result here
$('#form2').submit();
});
};
Post the first form without changing page, wait for the process to complete. Then post the second form. The second post will change the page, but you might want to have some similar code also for the second form, getting a second deferred object (post_form2?).
I didn't test the code, though.