When a first access node is considering setup of dual-connectivity service for a UE, the first access node could take into consideration the MU-MIMO grouping efficiency respectively of each of one or more candidate second access nodes, in order to decide whether to set up the dual-connectivity service for the UE and/or to decide which of the multiple second access nodes to use for the UE's dual-connectivity service. MU-MIMO grouping efficiency of a given access node could be a representative count of UEs that the access node has provided with MU-MIMO service per unit time. Thus, for instance, the first access node may decide to use a given candidate second access node for the dual-connectivity service of the UE, with the decision being based on the given candidate second access node having a higher MU-MIMO grouping efficiency than one or more other candidate second access nodes.