We prove that the existence of a \(Z\) -positive and \(Z\) -critical Hermitian metric on a rank 2 holomorphic vector bundle on a compact Kähler surface implies that the bundle is \(Z\) -stable. As special cases, we obtain stability results for the deformed Hermitian Yang–Mills equation and the almost Hermite–Einstein equation for rank 2 bundles over surfaces. We also provide examples of \(Z\) -stable and \(Z\) -unstable bundles, as well as \(Z\) -critical metrics, away from the large volume limit.