Large-conductance voltage- and calcium-activated K (BK) channels are key physiological players in muscle, nerve, and endocrine function by integrating intracellular Ca and membrane voltage signals. The open probability of BK channels is regulated by the intracellular concentration of divalent cations sensed by a large structure in the BK channel called the "gating ring," which is formed by four tandems of regulator of conductance for K (RCK1 and RCK2) domains. In contrast to Ca that binds to both RCK domains, Mg, Cd, or Ba interact preferentially with either one or the other. Interaction of cations with their binding sites causes molecular rearrangements of the gating ring, but how these motions occur remains elusive. We have assessed the separate contributions of each RCK domain to the cation-induced gating-ring structural rearrangements, using patch-clamp fluorometry. Here we show that Mg and Ba selectively induce structural movement of the RCK2 domain, whereas Cd causes motions of RCK1, in all cases substantially smaller than those elicited by Ca By combining divalent species interacting with unique sites, we demonstrate that RCK1 and RCK2 domains move independently when their specific binding sites are occupied. Moreover, binding of chemically distinct cations to both RCK domains is additive, emulating the effect of fully occupied Ca binding sites.