What do you call two functions that go together (i.e. as opposites), especially in programming

Inverse Function/Operator/Procedure

Inverse function

Inverse operator

Inverse procedure

In mathematics, an inverse function (or anti-function[1]) is a function that "reverses" another function: if the function f applied to an input x gives a result of y, then applying its inverse function g to y gives the result x, and vice versa, i.e., f(x) = y if and only if g(y) = x.[2][3]

https://en.wikipedia.org/wiki/Inverse_function

I see no reason you couldn't extend this idea.

EDIT

As pointed out by @Pax, "operator" may be a better term than "function" in this case. I've also added "procedure"