What I call the "ensemble" here is essentially in line with Wikipedia's definition of the theatrical term "ensemble cast:"
An ensemble cast is made up of cast members in which the principal actors and performers are assigned roughly equal amounts of importance and screen time in a dramatic productionIn MARK OF THE VAMPIRE, the illusive vampires Mora and Luna are an ensemble of two: although Bela Lugosi was the actor with the greater star power, Carroll Borland's Luna is equally important to the story insofar as both cultivating the vampiric hoax and symbolizing the incest-theme suggested by the main plotline.
It's also possible for some films to have an ensemble of two in which the characters are opposed to one another yet are potential dangers to the normal viewpoint characters, as seen in 1934's THE BLACK CAT. Here Lugosi's Verdegast and Karloff's Poelzig are deadly enemies, and their contest is the main plot of BLACK CAT.
Verdegast is nominally more sympathetic than Poelzig, but the dynamic between them is not one of "hero" vs. "villain," where it's generally true that either the hero or the villain is the imaginative center of the narrative. If anything their contest seems closer in spirit to the contest of two monsters, one benign and one malefic, where their competition bids fair to destroy innocent lives, such as we see in more extreme fantasy-films, like the 1968 "kaiju" flick WAR OF THE GARGANTUAS.
As my title implies, it's standard practice that features focused on teams of heroes-- like the AVENGERS-- or even villains-- like THE SECRET SOCIETY OF SUPER-VILLAINS-- have "ensemble" centers as well.
With dramatic programs, it can be argued whether or not a villainous forces is part of the "ensemble" or not, as with Alexis Colby on DYNASTY or Benjamin Linus on LOST. I would tend to say that in these cases the villain is opposed enough to the "good" heroes that he is more a regular cast-member than a part of the ensemble. But I'll reserve judgment as to other permutations.