I looked and as far as I can see we don't have a wiki page for em yet. But as far as I understand, it's basically alternate universes being pushed together, and Earth is the point where they collide, so one earth must be destroyed for the other to survive, I follow that much.

What I don't get is the weird dimension they go to, the place where Black Panther first met Black Swan, and where cap used the infinity gauntlet in issue 3. At first I thought that was the other Earth, but then Cap used the infinity gauntlet there so it must be this Earth....

Anyone know?

It's essentially the overlap between Universes as they collide. It's their Earth, but it's where they can see the other Earth heading for them because the universes are overlapping. Each time it was with a different universe, but because it's the same type of cosmic overlap I'm guessing that's why they're both a weird red pocket dimension.

Just finished reading it and as far as I can tell Monarch here is right, some kind of overlap space, which would explain why the IG worked, but it's power was not absolute in the way it was when Thanos used to wield it. Killing the gems doesn't leave the Illuminati with many big guns to fire so Hickman's gonna have to get very creative from here on out. The next issue teased at Galactus coming into play, I love big G but don't see how he can succeed where the Gems failed. Great stuff so far though, can't wait for the next issue.

Wait, they werent on our Earth the entire time? Until the last few pages of course