The old bees , the foragers don`t love another queen. But the young bees - nurse- don`t make trouble.
Usually I add frames from other hives,but I spray them before. A bottle with pump used for clean windows and filled with...I use some local alcohol drink, made from plums, strong smelling, but diluted with water. Essential is to bee smelling.
Spray both hive. The bees don`t recognize who is stranger.
Another method. Brush the frames added and put them in the hive. In a box brush bees from 3 hive. (more than 2)
Little secret... from 2 hive, they do a battle. More hives...peace.
Live alone the box for a half hour and the foragers is gone home. The nurse don`t know the way. Now shake nurse in the new hive.
For my, simply the best. Smelling both and go away.