Thank you guys, I got it finally using code in a custom template.
Have a nice day.
What I require is fairly simple:
Show a form, click next, insert data, then show a user registration form.
You are suposed to go back/next from one form to the other.
Then the user should be able to modify some data of the first form.
For what I need I could use code outside WP, but I need WP user management for security. And it’s not like I need to do lost of ajax calls.
I really wonder what’s the easiest way to do this : /