If it's just username/password, you can check that easily when the page first loads unless they are entering it all on the same page and if so...

You could also use hidden textboxes on your 2nd page and populate them from the posted data on the first page. Then just add a window_onload function that causes a form.submit(). Basically duplicate the structure of your original form, but do it with hidden elements on the 2nd page.

That way you can stop the processing if the business rules are not all followed.