Form validation in different controllers

I asking this because in the documentation (that honestly is the best one in comparison to all other frameworks) there is the only one example, that shows how to use form validation on the one controller - both rendering and validation (using IF statement to discern if form has been sent or have not), but there is not a word about using it in different controllers or at least different functions of the same controller (and i honestly think for this example it makes absolutely no difference).

And code in controller can quickly become a complete mess if you write everything in one controller, especially if your page contains more than one form.