There's no way of doing it seamlessly in PDF forms, as far as I know.
I've done it!! You just edit the custom keystroke script and enter this:
if ( event.fieldFull || event.willCommit )
this.getField("name of overflow field goes here").setFocus();
Only problems I have run into is that you cannot have the fields linked and there cannot be any special characters or spaces in the field name.
How did you handle following issues?
- user deletes text
- user changes text
- user paste text from other places
Also, note that that the last character typed into the first field will be dropped when the focus is set to the second field. This and the things mentioned by Bernd are what try67 meant by his comment of "no way of doing this seamlessly".