The term two-step ko is used in two different ways in English. Because of this dual meaning, on Sensei's Library, one is advised to avoid the term, since for both meanings there are less ambiguous alternatives.
The first meaning is a ko where taking the ko produces another ko mouth, which must be taken before winning the ko, as shown here. For this type of ko, see two-stage ko.
The second meaning is a ko where after winning the ko, one will have to make an approach move and then fight the ko again, which is the situation Black finds himself in in this diagram. For this type of ko, see approach ko. This is also called a one move approach ko, because Black must make one approach move in order to win the ko.
Please, if you see a page on Sensei's Library use the term Two-Step Ko, change it to Two-Stage Ko or Approach Ko (whichever is appropriate).
The second usage was coined by James Davies in the book Life and Death (1975), and from this usage came into much use in western Go. The first usage has been introduced later, on grounds that the terminology is a more correct translation of the Japanese terms for these shapes.