It sounds like you want to set up two fields where text entered into one will overflow into another when the first is filled. If that's the case, stop, back away slowly, don't make any sudden movements, and maintain eye contact with Acrobat at all times. Theres' no way to make this experience a good one using just form fields in Acrobat.
Writing a script to jump to the next field when the first is filled is relatively simple but then yo have the problem of selecting text across two fields when the user decides to edit the text they've entered.
A better approach is to create a dialog that will get triggered when either field is clicked (mouse down), allow the user to enter as much text as they like, then split the text across the two fields when the dialog is dismissed. You'd then populate the dialog with the existing text when they clicked again, then split it after another edit.