Does Karthus's passive kick in before or after Yorick's ultimate goes off? Or does the Death Defied work both times that Karthus "dies"?
In a patch a few months ago they established an order of operations for all of the post-death effects. I'll try to find it and link it here, but I know that these 2 effects occur in this order:
I can only find a link to the Riot patch where they define the order for death preventing effects.
On the LeagueOfLegends Wikia, we have an order defined for after-death effects which appears accurate to me.
Yorick ult will come first so that Karthus gets a few extra seconds of being alive before he dies and enters Lich Form.