Not too sure, I'll have to try it out on my alt later when the cooldown for using my main is over.
(trying to at least get the hime on my alt)
edit: Tried a few times and didn't get both to show up using Apocalypse and Diabolos as mains on separate party set ups. Only Apocalypse showed up.
And same vice versa with my alt using Ouroboros and Diabolos on separate but only Ouroboros showing up. Might prioritize highest level?