OpenEmu to Desume

Hello, I just started using emulators this past week to play pokemon and I love it. However I installed openemu on my Mac and my brother who now wants to play has Windows. From what I understand the only easy way we can battle is if we both run Desume, if I'm wrong at this point please tell me how we can interface desume and open emu, if not then I continue. If i get the Mac version of desume, how can I keep the save file I already have of my ROM for that specific game, or do I need to restart? Followup, at this point how do we trade and battle together via desume? Thank you in advance, any help is much appreciated!:artist: