Does anyone know if these two are suppose to work together? I have both and Howl From Below appears to take precedence, but I only ever get 1 skeleton spawning. 
Yes, they work just as expected when both are equipped. Black River lets you consume up to 5 corpses to increase the effect size and damage while Howl from Below makes your Corpse Explosions run at targets and gives you a damage multiplier.
Your misconception might be that Black River does not give you up to five corpse explosions at once but one corpse explosion with a greater effect.
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Damn. It would have been awesome to see up to 5 skeletons spawn and run at enemies. 