To the best of my knowledge there are no rules in this situation.
I was trained to try to leave both aircraft at their preferred levels and use radar headings to get them passed each other. Only if that is not possible (e.g if very busy) I will apply my own 'rule' of whichever aircraft has furthest to go stays at its level, the other descends and can climb back to the level later if it wants to.
In Europe there seem to be some countries where ATC generally resolve conflicts vertically and others where radar headings are the preferred solution.