Are you sure it is the queen that is the problem? Even good queens need a minimal amount or workers to support her and be productive. If there are not enough resources and nurse bees to take care of the amount of brood she must lay to make the hive grow, they will just putter along. Sometimes just adding a frame of capped brood give a slow hive the kick it needs to start growing.
My opinion is combining is just giving up. Your hive numbers go down by one and the bees you add to the other provide minimal difference (unless of course you are combining 2 weak hives).