This is a simple achievement. The only thing that might be tricky for some is figuring out where to get the 1966 Batman characters. You need to play the "Same Bat-time! Same Bat-channel!" level to get Batman (1966). Refer to the solution for the achievement of the same name for the location of this semi hidden level.
Once you have Batman (1966) press
in any hub to load the level select screen and start the 75th Anniversary level in Free Roam. Once it is started destroy all the tables, build the dance floor, and set both characters as Batman (1966) just to be safe in case you switch between the character since the floor can be touchy (aggravating at times), and complete the dance puzzle.
This can be completed for two players at the same time as long as both are set to Batman (1966). So grab your friend, child, or significant other and get you both the achievement.