How can I add a feature to a part in an assembly with a reference to another part of another assembly?

Here's my situation:

I have an assembly(assy.sldasm) that contains 2 other assemblies (test.sldasm  & test2.sldasm).

I need to add a hole to a part in test.slasm that is concentric to a hole in test2.sldasm.

but I'm receiving this message.

according to this page I have to unlock the assembly, but it seems the assembly is already unlocked.

A sample file is attached.

check the attached jpg file too.

Any kind of advice is much appreciated.