Rationale (February, 2017):

The effects of such attributes are implementation-defined.

P0028R4 contains this example:

  [[ using CC: opt(1), debug ]] void f() {} // Same as [[ CC::opt(1), CC::debug ]] void f() {}
  [[ using CC: opt(1)]][[ CC::debug ]] void g() {} // Okay (same effect as above).

However, there appears to be no normative justification for the claim that these two attribute-lists have the same effect.