My strategy was to keep focusing on one of them and kill him off before another showed up. I have to say this was one of the boss battles that I was able accomplish in only one try. Luck, I think. Because I never got the one hit grab and kill move they can do. I stayed in close and just kept pummeling him with the combustion fire spells untilled they died.
That said, If I were tanking it I'd make sure to get some good magical defenses and maybe try to do it fully human so I could bring some phantoms in on my side. I'd also say equip Havel's ring to move faster (Havel is at the bottom of the watchtower basement by the door that leads to Darkroot Basin) or maybe that ring that boost magical defenses (sold by the merchant in Sen's Fortress). The moment you see the first King, run to him. The quicker you engage in battle the more time you have to defeat one of them before another shows up.