Double Screen Board
The double screened boards are used to stack two different colonies into a single hive. The double screens keep the workers from both colonies separated, so they cannot kill each other.
One of its uses is to help a weak colony survive winter. A strong colony is set at the bottom, then you install the double screen, and you place the weaker hive on top. The two colonies will share the smells and get used to each other and the stronger colony at the bottom will provide some heat to the weaker colony at the top (during winter).
It comes in 8-frame and 10-frame sizes.