、。
She has two cats — one is white and the other is black.
Literal
She [topic-は] cat [object-を] two-counter is-keeping, one-side [topic-は] white and other-side [topic-は] black is.
Two grammar points. 二匹 uses ~匹, the counter for small animals (cats, dogs, fish, insects). 一方 / もう一方 ('one / the other') is the standard 'one X, the other X' contrast — 一方 alone simply marks 'one side'; もう adds 'another, additional.' The である copula gives a slightly more formal/explanatory feel than だ.