Considering that the data from the first form cannot be saved yet
because of validation, all you need to do is to add hidden fields that
relate to the attributes from the first form.
Say the first form is to change name and the second to change address,
you would add in the second a hidden field that uses @user.name.
This will work if you are using @user = User.new(params[:user]) to
initiate the user object on every form.
It will get nasty when you try to validate the user though, cause he
will go trough all forms without errors and on the last one might get
an error that "Name" (from first form) is blank.
My advice is to add all fields who need to be present for the model to
pass validation on the first form. Considering a user object, this
would usually be username, password and email. Then you show the user
some reminders to update his address in case he have an empty one.
Remember, make the "sign up" forms short and quick. Ask the user to
fill additional but not that critical data later...