1 way is to take a brood frame from a strong hive and give it to a weak hive, add smoke to allow them time to adjust without knowing/smelling that there a new bees in the hive.
Another way is to swap the position of the weak hive and the strong hive. Returning workers go back to the original location. If the hives are originally side by side and look totally different, the bees will not be fooled. They will pile up at the old location and then find there old hive. If they are at least 5-6' apart they will probable go to the new hive. Ask me how I know this? :-D