Uniting

Joining two colonies together, or 'uniting' is done for various reasons, a colony has become queenless and there are no spare queens available, or it is too late in the season for the colony to build before winter.  uniting may give one large colony a better chance of going through winter rather than two medium colonies.  Re-uniting after an artificial swarm, 'AS.' Or the reason I usually unite is to absorb swarms into my apiary.
All the swarms I collect are treated for varroa and isolated from my other colonies, they go through at least one brood cycle to make sure they are disease free, they will then either be requeened or united into one of my colonies, depending on time of year and availability of new queens.  It has been a particularly difficult year to produce good quality well mated queens, so just about all the swarms from this year are being united into my overwintering hives.

This is the way I unite, it is not the quickest and not the way most books or other beekeepers will do it, but it has over the years been 100% successful for me, with minimal loss of bees in the uniting process.  On the right in the first picture is a working colony with a Queen in, Queenright, Q+ are the ways to show a colony with a Queen.  On the left of the first picture is one of this years swarms, the Queen has been removed.  Between the two hives is a crownboard that has been modified to work like a Snelgrove board, the feed holes have been covered in varroa mesh and there is an adjustable opening in one side of the board.  It isn't a Snelgrove board exactly nor is this the purpose a Snelgrove board was meant for, which was a form of swarm control.

The board is placed directly on top of the
Q+ brood chamber.

The Qless brood chamber is now placed directly on top of the board with the entrance open and at right angles to the Q+ entrance.  The bees can smell each other through the mesh but are not able to do each other harm and they can remain like this if necessary for weeks.  The pheromones from the Q+ passing through the mesh seem to suppress the urge for the Qless colony to produce a drone laying worker, or at least I have never had a colony in this situation produce one.

Once the two colonies have had a few days to get used to one another I replace the board with a thin piece of card that has been slashed with a craft knife.

It takes the bees a couple of days before the bees eventually break through and by this time are already one colony.

You can now either run this as a double brood colony or shake all the bees into one brood,  This will depend on time of year, weather, and your own preferences.

I normally keep very large single brood chamber colonies, I manage my bees to be on 11 full frames of brood throughout the summer.