Hussar said:
I've never said anything about LE and CE always working together. All I said was that pre-Bloodwar, nothing in the rules really said one way or the other. They could or they couldn't as the DM decided.
It's likely that I over-stated my point there. Rather than state it categorically, I should have given a caveat. So let me clarify.
Typically, you will not see a vrock and an erinyes working together. Demons and devils have different agendas and different goals and these goals are ultimately mutually exclusive. If demons destroy the world, the devils will have nothing left to rule, and if devils control the world, the demons will be unable to destroy it all, and so their goals are at cross-purposes. A given game may give the two an allegiance, however tenuous and fraught, but the differences between the two assure that they largely remain in different camps in most D&D games. It's not an absolute prohibition, merely a tendency.
Yugoloths in MM2 appearance are stated to work with both demons and devils. This is a notable thing about them, because of the aforementioned difference in goals and tendency to work at cross-purposes. This implies that if the yugoloths have any goals, it includes both the demons and the devils getting part of -- but not all -- of what they each want.
In some games, this rivalry between fiends isn't very important: their allegiance to Evil trumps their differences. In this case, they likely still retain independent operations, and different attitudes, and likely still have tense relations, due to their very different outlooks, actions, and goals. Yugoloths may be the glue that holds the team together, the
Team Mom for Team Evil. Dragonlance might be a good example of this kind of setting.
In other games, this rivalry has exploded into genocidal wars. Yugoloths here may serve as traders, arms-dealers, and neutral parties between the sides of the conflict, working with both sides. Planescape might be a good example of this kind of setting.
In any game that employs these alignments, the division between creatures that are Chaotic Evil and creatures that are Lawful Evil is real. What varies is the intensity of that division. Even at its most amicable, it's still strained. Even at its most acrimonius, it's still not categorical.
Hussar said:
Post Bloodwar, you cannot have them working together, full stop.
That's not accurate. It's not accurate to talk in binary terms like "cannot." Even in Planescape, there's examples of demons and devils who have -- at least momentarily -- laid aside their hatred for other purposes. Heck, since everything canon in Dragonlance is also canon in Planescape,
there's an example right there.
No setting is really monolithic can/cannot.