A mechanism for controlling configuration of dual connectivity for a user equipment device (UE), where the dual connectivity includes the UE being served cooperatively by (i) a first access node over a first air-interface connection between the UE and the first access node and (ii) a second access node over a second air-interface connection between the UE and the second access node. An example method includes (a) predicting whether both (i) in the dual connectivity, a data split will put a threshold greater portion of data flow of the UE on the second air-interface connection than on the first air-interface connection and (ii) coverage strength of the UE from the second access node will be threshold lower than coverage strength of the UE from the first access node and (b) using the prediction as a basis to control whether or not to establish the dual connectivity for the UE.