If you receive a secondary from a school that asks you to elaborate on say, a volunteer experience, already described on your primary, will an inevitable overlap of primary content on your secondary be ok?
I don't think there's anything wrong with referencing your PS. Sometimes it's awkward writing about an event or situation in your secondary that requires more elaboration which may be present in your PS. Rather than assuming the reader has already read the PS, it would be appropriate to just reference the PS for further elaboration. The reader will likely read both, but who knows if they read the PS or secondary first. I believe it's better to reference your PS rather than wasting space, sacrificing flow, or assuming a reader's knowledge of your PS.