you can somewhere find the modpack for reborn on this forum, just google it and you should find it. The mod you need is the sharedbox one. It will turn your last box into a shared box, meaning that if you put a pokemon on one save in that box you can pick it up from another save. I am not sure if it is an additional box or if it replaces the last one, so you should empty it on both files before including the mod in your data folder of the game.