To facilitate installing a blank bar behind your active colony, you can use a follower board. A follower board is anything that can be placed inside a Top Bar hive that will stop bees from entering the unused space behind the last occupied bar. If you have a follower board, place it behind the last bar in the colony so you can stop the bees from interfering during the process of combining.
A follower board stops bees from entering unused space.
Drape a few sheets of dampened newspaper over the blank bar and form it to the sidewalls so no bees can get around it. If you used a follower board, remove it and push the bar with newspaper so it joins the last bar of the colony.
You can now proceed to install the weaker colony into the stronger one by removing each comb from the weaker colony and placing it behind the newspaper-draped bar. The bees from both colonies will eat through the newspaper and, during the time it takes to join, will have acquired a similar scent, thus minimizing fighting. After three or four days you can remove the bar with the newspaper, and you are done.