A method and system for controlling handover of a user equipment device (UE) in a system including RAT1 access nodes and RAT2 access nodes. An example method includes, while the UE is served with dual-connectivity cooperatively by a first RAT1 access node and a first RAT2 access node, (i) using a comparison between spectral efficiency of the first RAT2 access node and spectral efficiency of the second RAT2 access node as a basis to set a measurement threshold to be applied by the UE for evaluating coverage of a second RAT1 access node for possible handover of the UE from the first RAT1 access node to the second RAT1 access node, and (ii) causing the UE to apply the measurement threshold for evaluating the coverage of the second RAT1 access node for the possible handover of the UE from the first RAT1 access node to the second RAT1 access node.