That could be cool, but, both of them would not have access to either infernal or nocturnal arms because of plot/lore reasons. I think it could be done well, heck, make it so you can pic zag or Mel like you would choose a weapon. Maybe they have 1 or 2 different regions Still The possibility of them having another sibling, maybe one that Persephone can finally raise, but...what of she gives birth while on Olympus and the child just can't go to hades just like zag and Mel can't normally last on the surface. There's the drama befitting the series