Combined diff with name-only option

Hello,

When I run:

git diff-tree --cc 511f273b99af4529f6eb30069bb9070dc73fbbb4

As expected, I get the diff for the files that are changed in both parents.

When I run:

git diff-tree --cc --name-only 511f273b99af4529f6eb30069bb9070dc73fbbb4

I get all the files listed including ones that are changed only in one 
parent.
I would expect to see only names of the files changed in both parents 
like in the case when --name-only parameter was not present.
Is it a bug or is it done by design?

BR,
Janusz.