I was refering to a Klinklang deck, which is the reason I said they co-exist. You wouldn't put Cobalion EX into any other decks, because you can't splash him like Registeel EX, they can't co-exist if they aren't together. I wasn't talking about splash-ability. As for the Kyurem EX better than Groudon and Reshiram EX's. What? Kyurem beat them because of weakness, I don't think Cobalion EX hit's Registeel EX for weakness.
Registeel EX is more splashable, which makes him better thaan Cobalion EX, can spread, which Cobalion can't do, and resists more damage from attacks, but, only resists more if you're using his second attack, which is normal with Klinklang. Coblaion EX is better for a few reasons, One-shotting Bouffalant and Sigilyph, 2-shotting everything, getting rid of Blend, Prism, DCE, and Plasma energy, for one energy. Cobalion EX can actually do more damage, tank more, and KO more than Registeel, BUT isn't splashable like Registeel.