html can't do this, you have to use a higher level language like PHP or Javascript.
But normally code like that is only used maliciously, if forms were meant to be linked they would share a common database or have builtin handlers for interacting
The only legitimate use for code along these lines is a post-echo, which is used by paypal to authenticate transactions. That works by making a http post back to the server containing the same data as was just sent to the client, if the server returns VALID it is a legitimate transaction.