Once submitted from you original form, go ahead and execute the code needed to generate your email message. Now depending on what kind of data was submitted from the first form you could just append the form data to the query string and redirect to the new page....something like this:
$location = "script.php?var1=" . $_POST["var1"] . "&var2=" . $_POST["var2"];
header("Location: $location");
You could also create a new form on this page and just set all of your input fields to type "hidden" and use javascript to automatically submit the form when the page loads(using onload in the body tag)
What are you doing with the values in the next script? Would it be possible to write them to a database here and then grab them in the next script?