I've been in enough hunting trips with Rhysus and Daevos during Kethuru type of events to know that they can get along quite well if they share a problem.

Well, that's facing Kethuru's destruction, which is completely different to a "general scenario". And I don't think forcing the Communes, which have a common ground, together is nearly as bad as trying to force the cities, which really have no common ground other than "they have more buildings!" together. It's just silly if you expect it to be for any more than a tiny length of time.

Just commenting on what someone said, that Celest and Magnagora could never work together even facing civilisation's destruction.

Oh.

For some reason I read the original post as "except." Nevermind.

United communes assault on both cities cannot be compared to Kethuru's second coming...
Unless you'd have the means to really obliterate the civilization, I stand by what I said.

Sure, players can pull off basically everything, even New Celest and Magnagora being buddies. But they -shouldn't-, even if both were at war with the same enemy. They just... contradict each other, fact that they are cities alone is no base for agreement, everything else proves they're anathema to each other.

The same could be said about the communes, in the end.

And, maybe Kethuru was a bad example. We had the same when the Psionics skills came out.

So which is it?

Both communes want the fae alive? Fine. It's not the case with cities. Magnagora certainly doesn't want cherubs or angels to stay alive.

Both is it. The two communes are opposed just as much as Magnagora and Celest are, but here's an opportunity to meet on common ground.

Both cities want their version of the "converted" fae to stay alive, or at least their roleplay background suggests it, whether they unite their efforts on that common ground or not is up to them really.

Here's how I understand it.
Both communes want the fae not to be converted first, so it's their common goal.
Cities want converted fae to stay on their own cosmic plane, but not the other city's. I don't see a common ground here.

There's only one Faethorn which is precious to both communes. Cosmic planes are completely separate and opposed.

If there's a flaw or falsehood in that logic, please explain.

Magnagora and Celest can both attack Faethorn without being allies. Heck, they'll be converting Fae and killing the opposing citymates at the same time, even better.