It is possible to do that using Javascript.
Supposing you have two text fields in the same subform, named TextField1 and TextField2, you can put a script in the change event of TextField1 field to check the number of characters typed and to set the focus to TextField2 field when you type a character beyond the limit, putting this new character in the TextField2.
See an example of this script below, using a 20 characters limit:
