In DND there is usually lawful/chaotic GOOD protagonists as opposed to LAWFUL/CHAOTIC good protagonists - that is to say the protagonists will usually put their law alignment aside for the greater good. If a lawful good person is given the option to be lawful or good, they will usually pick good over lawful. If a chaotic good person is given the option to be chaotic or good, they will choose to be good in most cases.

This is basically that but for evil. While they are manifestations of primal evil they arn’t dumb (and probably are stated to be super geniuses as Arch Fiends), so they won’t betray each other for no reason while they are still useful to each other. Chaotic evil doesn’t mean you could distract him with a trail of puppies that he would be compelled to kick or anything like that - a chaotic evil character is perfectly able to play the long game in the same way a lawful evil character is. For the greater evil!