The description here is a little misleading, as you actually get this ending by choosing to save Shaundi in "Three Way", instead of killing Killbane. However, after that mission, you'll take part in another mission, starring in a hammy space movie and killing "Killbane" in space.
You can get both endings and achievements from "Three Way" without having to save beforehand, as the game allows you to play it again (unlike the end of Act 1's split achievement with "The Belgian Problem").