They aren't physically approaching like that, It's more of a weakening of the walls between the two universes that contain them.
The positions are similar enough that a kind of portal- or just an weakened area opened up in the skies between the two worlds, so that each would seem to be floating above the other.
Anything with enough power and focus can jump from one world to another- Gods would find this easy, though groups of powerful mortals (Mages and heroes) could also attempt it.
One of the first visitors will be a bunch of mythical beings that are trying to get off the Other World.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather