It is not commonly used because while it increases honey production, the gains are not enough to offset the management complexities.
Pros:
You double the honey yield (but it is approximately the same as what each colony would get on its own)
Cons:
If you lose one of the queens, you will have a hard time getting her replaced as the workers will not start cells due to the other queen's pheromones. They will also kill a queen in a cage as they already have an active one.
If the queens are not "equal", the bees will drift towards the more "attractive one"