First off, you're fighting two wars, one with Halfdan, and one with Ivar. You only need to win one of the two wars to survive.
Second, you need to survive long enough for other Catholic powers (aka everyone else) to join your war. Thanks to the increase in the number of territories from Holy Fury, more territories means more Catholics, which means more Catholic troops. Also, more allies means you get to see more of the map and can plan deployment better.
Third, yes, if you capture Halfdan or Ivar, that's an instant win, congratulations, but that's rare.
I'm most familiar with the Great Heathen Army as Ivar, but I have lost to Northumbria before. However, that's usually more of the "cut my losses now and save my event troops for blitzing across Ireland instead" strategy, that the AI might not consider. I'll give it a go as Northumbria and see if there's anything more I can recommend.