[NTG-context] Headertexts in the center

Hi there, with \setupheadertexts which takes four arguments, one can place headertexts on four places on a double-page. But how one can place only two headertexts at the top-center of the even/odd pages? So the arguments should not be places (left-right)-(right left) but simply (center)-(center). Thanks! Huseyin