Ability of DFF to be edited based on another DFF value

We have a requirement to in DOR page to set a new DFF value as read only based on another DFF value. We are trying to get the value of the DFF using the expression #{bindings. dbTfzbKlassebAusgabe.inputValue} but it is not working as expected, the DFF has value but it’s throwing as null.

Sample Scenario:

DFF 1: TFZB Klasse Ausgabe

DFF 2: TFZB Status 2

If DFF1 has a value, the DFF2 will be read only, else if DFF1 is null then DFF2 will be editable.