Form within form?

So I have a form made, in the middle of which I have group of inputs I'd like to go to a separate location if the submit for that group is pressed.

I know a form within a form is invalid HTML; I'm curious if there are methods linking two forms, or if I should simply look at restructuring the page to make what I want to do work.

You can have a multi page form if thats what you're asking.

Server side language to pass form data on through the process.

Well, I know I can create a multi page form (though the only technique I know of doing it is to store all submitted data into a session, and carry along... are there other techniques to do it?), but I'm asking about a form split up on the same page, eg:

<form method="post" action"url">
// Form stuff here
</form>
<div>
// AJAX form stuff here
</div>
<form method="post" action"url">
// Form stuff here
</form>

And have the stuff from both forms submitted when one is submitted.