Find the object of a field widget on a form

I have two fields on a form. One is a select list with an Ajax callback on ‘change’. When I am in callback, I want to call a function (addMoreAjax) on the second widget. For that I need to find the object that represents the second widget.

I can get the form object from the form_state, but I am unable to get the widget objects.

Any hint available? Many thanks for your support.

Regards
Christophe