Well I may be new at his but so far it seems like we have two separate projects here. On the one hand you have the zombie attack but on the other, this jumping between worlds. I don’t think we need to keep the two worlds contiguous per say. How about this: The attack from space happens in the fantasy world yet the normal world is fine. This could further add to the sense of confusion of the main characters when they first start their journey and make the first jump. That opens up whole realms of options, maybe the parallel world is almost the same as ours but with historical events happening just slightly off, then the meteors hit and all bets are off. Maybe the meteors happened hundreds of years ago and humanity has adapted to their new set of circumstances. Sorry to jump the gun on the first day but that’s just the vibe that I’m feeling from all this. feel free to expel me if Ive completely botched it :).