There is no user-level interface to the 'commutes-with-conjugate'
mechanism.
Maybe there should be:
(%i2) :lisp (setf (get '%derivative 'commutes-with-conjugate) t);
(%i3) conjugate(diff(f(x),x));
(%o3) 'diff(conjugate(f(x)),x,1)
(%i4) conjugate(diff(f(x,y),x,5,y,8));
(%o4) 'diff(conjugate(f(x,y)),x,5,y,8)
But maybe Edwin wanted to do diff conjugate --> conjugate diff.
By the way, diff conjugate = conjugate diff isn't an identity.
Barton