This is acheived by finding a hole in the floor in the dungeons. These are placed in the dungeon in a similar manner to the traps normally found in the dungeons. If your Pokémon goes into the hole, the Pokémon behind it will jump on.
You can keep doing this until all of your Pokémon are up on a tower. This means the four of your Pokémon are moving as if they are one and they will be able to attack simultaneously.
To get an optimum tower, you will have to tower your Pokémon up based on size with the biggest at the bottom.