To split a field in case overflow ....

Hi all,

My form got a text field, say A, multiple line input which is not limited such that a + sign may occur in case too many text are typed.

What I want is:
By pressing a button, those text will be split and part of it fill up B, and the rest all goes to C.
(A and B are of the same width and height but B is not scrollable).

On the button's click, I write JS that place each time 1 character from A to B and then immediately check something like event.FieldFull, but seems it doesn't work at all, is this property not using this way?

Thanks in advance.