Well, you can use: $HTTP_REFFER
Add this code inside your form page:
<?php
$a_url=="First URL";
$b_url=="Second URL";
if($HTTP_REFFER==$a_url){
print <<<html
<form method="POST" action="">
<input type="text" name="first_input"><br>
<input type="text" name="second_input"><br>
<input type="submit" value="Submit">
</form>
html;
}
elseif($HTTP_REFFER==$b_url){
print <<<html
<form method="POST" action="">
<input type="text" name="first_input"><br>
<input type="submit" value="Submit">
</form>
html;
?>
