Edwin Ashworth has already described the correctness better than I.
For two reasons, 'two-step' is the correct option. First, the word 'step' is here being used metaphorically – one might even say as a unit. Units rarely pluralise as premodifiers (a three-ton elephant / a six-mile hike). Second, attributive nouns are rarely used in plural form in any case (a sweet shop (not sweets shop) / a log cabin / an animal shelter).
But I wanted to add, it's not uncommon for methods or things to be called the "X Two-Step". So if your method is for squeezing a fruit to extract juice, you could call it the "Juice Two-Step". This is essentially a marketing technique playing peoples' familiarity with the dance moves.