Lith had only met two individuals with two cores and, even if he wasn't aware of it, both of them had been artificially created. The first was Kalla, her blood core was meant to expedite her turning into a true undead.
The second was Gadorf the wyvern, who had a black core to assist him in forcefully refining his mana core by feeding on the magical energies of others. Both artificial cores had been placed far enough from the natural one that it would not alter its functions.
According to Solus's mana sense, Count Arik Xolver's situation was completely different. The blood and the mana core were so close that they almost overlapped. Unlike Kalla's blood core, Xolver's wasn't stable and needed to drain small amounts of mana from the nearby mana core to sustain itself.