How to make BEX Customer Exit Variable inactive through Customer Exit Code

Hi,

I had created two variables VAR1 and VAR2 as Customer Exit variables

If VAR1 is entered then it should automatically make the VAR2 as NO Entry Variable.

vice versa also required.

can u help me with any code in CMOD so that we can make it inactive through Customer Exit Code.

Thanks in Advance.

Sunil.