did the 18 patch get released alread?
u cannot freeze them both without the ice rune, as stated in the guide, u need the ice rune to freeze them both. but you can freeze them seperately with the ice potion and help seth for his ending they both will trigger his ending one way or another, seth has 2 ending varieties
Ivar’s ending is the easiest to trigger so long as you chose truth and do not complete the diary quest or kiss him, wether Fia accepts your request to put Ivar first or not as long as u do those 2 prequisites, his ending should trigger