The basic problem is that the process of submitting a form from a webpage requires sending an HTTP request to the 'action' URL for the form. This means that the current page unloads and the browser displays the response to its HTTP request, so, essentially, the second form on your page no longer 'exists'.
Some PayPal shopping cart functions offer a callback notification that can be sent back to your website. That notification would show whether or not the user's payment was received, so you could have a script on your site that would send a message only if the payment went through properly. You might look into that. Good luck!