i am not sure i understand the question usually you have the lowpoly ready and bake the ao from the highpoly down to the lowpolies, like you would do with a normalmap.
People used to bake an AO from the low and the high and then combine them, but these days I think most people just bake from the high since the lowpoly and highpoly shapes are generally much closer to each other then they were back then. Plus there's SSAO which helps in eliminating the need for a separate lowpoly AO bake,…