What's worse than one Big Bad? Two Big Bads, possibly more, working in intentional collusion with each other. Sometimes, they will work together just fine, being all respectful and well-mannered, but more often than not, there will be rivalries between them, and they will tend to break out into a literal example of an Enemy Civil War.
A good rule of thumb for the health of the alliance is how well their Evil Plans mesh. If both of them want revenge on The Hero, then they will likely work well together. If both of them want to Take Over the World, then one of them may eventually say I Can Rule Alone. If one wants to rule the world and the other wants to destroy it, conflict between the two is inevitable. If their goals are entirely separate, then Screw This, I'm Outta Here is likely once one of them succeeds.
Not to be confused with The Dragon (a main villain clearly subordinate to the Big Bad), though if one of them becomes dominant he may reduce the other(s) to the position of Dragon with an Agenda, Dragon-in-Chief and/or The Starscream. May overlap with Unholy Matrimony. Contrast Big Bad Ensemble, where there are also two or more Big Bads operating simultaneously, but not necessarily working together or even interacting in any way, and Contrast Co-Dragons, where one Big Bad is directly served by two or more equally ranked Dragons. See also Villain Team-Up. See Evil Duo for cases where the villains fit two of the three roles (usually "superego" and "id") of a Freudian Trio.
