I wouldn't go with showing/hiding DIVs.. It's too problematic for me.
You can just make 3 actions (2 actions - for two forms and one to store data
in DB and redirect), submit first form to second action, and second form - to
third. When validation of first form in second action fails, just display
first template (the same with second form). All submitted data could be
stored in hidden fields so you don't need to mess up with sessions.