User blog comment:Miraka/Prediction of Chapter 385: Spirit King vs King of Hades/@comment-24403746-20140517103039/@comment-7418318-20140517104039

Marde Geer doesn't look that distinct from the other Tartaros members or even a normal Human that everyone would know him as the Demon King just from looking, so the Spirit King must have seen an image or heard a detailed describtion of him once.

That still doesn't mean they have to share a direct connection or even that they ever met face to face, but they must at least know a bit about the other.