"Pas" means "step", as others have said. Apparently "pas" as a negation in French was originally an intensifier of the main negater "ne", a bit like saying "I will not do it, not one step", and then gradually became required in most situations up until "pas" on its own came to mean "not".
But "not both" is "pas les deux" or "pas tous deux" and "pas de deux" would translate as "no twos" (say in a hand of cards).
And it's not a two-step, which refers to two steps by the same dancer. It's a longer series of moves by two dancers.
Source for the history of negation in French: The Unfolding of Language by Guy Deutscher. Great book.