[@BCTheEntity]

If they do appear together in a fight i was thinking a sort of mashup of both of their basic/normal fight themes.  If one of them is killed, it will immediately reset the fight as a 1 v 1 boss battle.  Unless you think that sounds like too much T.T [s]hardmode shadow is scary.[/s]

Honestly, it depends on what happens/how a character acts if i will have both of them attack at once or not.