If i was god...I would tell everyone to never go on the other side of the earth, telling them it's the most
deadliest place on earth. I would give the side with humans lots of water, meat, land to grow with, and
mountains for protection from the "other side". On the "Other side", I would have alien life forms go there
and see how it was, then speeding up the technology of humans, thus creating an all out war on aliens, IF they get curious enough.
TL;DR:make humans stay on one side, and aliens on the other side fight.