Use DFF in binding params of expression builder(page composer)

Summary:

How to use DFF in binding params of expression builder(page composer)

Content (please ensure you mask any confidential information):

We have created two DFF's on Debrief Header(RCL_DEBRIEF_HEADER_DFF). We want to make second DFF dependent on the first.

Use case:

If DFF1 value is NULL, we want to make DFF2 read-only. Else DFF2 needs to be editable.

DFF1 is number field(without any LOV) while DFF2 contains LOV.

We are trying to achieve this using page composer. However, we are unable to access DFF1 in expression builder binding params(screenshot attached).

Can you please confirm whether it is possible to use DFF's in binding params. If yes, what is the value that needs to be used?