Duo

Duo is (or are) a pair of spirits resembling cats, Oriental Shorthairs to be specific, who share a close physical and emotional bond with each other. The two came into being at the same time, and are capable of merging and splitting their bodies however they please. The white one is generally friendlier and more excitable than the black one, and they love to play-wrestle when they're separated entirely.

They sound like this video. The more aggressive-sounding one is the black one.