A cell site that has multiple first-RAT carriers will monitor a capacity demand respectively of each first-RAT carrier, with capacity demand representing an extent to which adding more service capacity could be useful in view of the state of service on the 4G carrier. Further, the cell site will dynamically allocate resources of a second-RAT carrier for use in dual-connectivity service of UEs served on various ones of the first-RAT carriers, with the allocation being based on the capacity demands of the various primary-RAT carriers. For instance, if a first first-RAT carrier has a higher capacity demand than a second first-RAT carrier, then the cell site could allocate a more resources of the secondary-RAT carrier for use in dual-connectivity service of UEs served on the first first-RAT carrier than for use in dual-connectivity service of UEs served on the second first-RAT carrier.