A method and system for controlling application of split-uplink mode in a wireless communication system including an access node. In an example implementation, a method includes determining a first count defining how many user equipment devices (UEs) are connected with the access node and do not support a split-uplink-mode operation in which uplink user-plane data flow is split between air-interface transmission to the access node and air-interface transmission to another access node. Further, the method includes determining a second count defining how many UEs are connected with the access node as part of dual connectivity and support the split-uplink-mode operation. And the method includes, based on the first count and the second count, controlling whether the access node will allow the split-uplink-mode operation, such as whether the access node will allow new activation of the split-uplink-mode operation.