Earth-Zero has elements of Earths 1, 2, 4, S, & X. So if Earth-Zero were to "split back" into its "component" earths, that should be at least a 5 way split. That cover picture looks more like two earths colliding than one splitting into two.
If that is true then, what is the other Earth and where did it come from?
You have not been following the Elseworlds lines have you?
From a bit in Previews catalog, Infinite Crisis is supposed to visit the worlds described in Kingdom Come and Justice League: The Nail.
Kingdom Come was a possible future.
The big thing here is Justice League: The Nail. In the series with the Limbo Cell, it was shown the the Nail universe is actually a multi-verse. My guess is that if that is a collision between two Earths, the other one has to be The Nail Earth. Nail and Zero are oppisite sides of the coin. There is also the posibility it is the Wildstorm universe with Mister Majestic and WildCATS. Captain Atom is now in the Wildstorm universe
It's not like a few Earths didn't escape damage. Earth C and Earth C Minus were stated to be too far away for COIE to affect