Enabling textbox2 as data is entered in textbox1

Question

how to enable 2nd textbox as i enter data in first textbox.

i have two textbox  txt1 and txt2. txt2 enable="false".
i want that as i enter any character in txt1 (focus still on txt1)  txt2 enabled property should become false.

i am able to do this when i set my focus out of txt1. but nt able to do this as i enter the first input in the txt1.