[phenixbb] Refinement of two co-factors at the same position

Dear all, I'm trying to refine the occupancy and position of two co-factors iron (III) and FEO (Fe-O-Fe) in a crystal structure. The iron (III) is placed with some occupancy on one of the iron positions in FEO. If I refine them together in Phenix with low occ phenix.refine moves the atoms apart. What is the best way of dealing with this problem? Thanks and best wishes Kornelius