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Is this viable? I've heard a few suggestions of lists that run both in a big ol soup army, but would it be points efficient? I suppose if they have a changeling, maelific lords and brims it would look pretty nasty.

God damn soup

You answered your own question really..

Could even do Mortation, Magnus, 2 knights etc. The answer is yes this will most likely be a thing.

Magnus + Mortarion + Changeling + 2x units of Deathshrouds and Brimstone filler.

Magnus can get a 3++, re-rolling 1s and has a -1 to hit.
Mortarion can get a -1 to hit and can throw any high damage shots onto the Deathshroud.

Mortarion will cut through hordes and Magnus will destroy big things. Magnus can also throw down Warptime to get Morty where he needs to go.

I wrecken they'd be a pretty nasty double team.