The Reality Splitter is a technology which, when utilised, can “split” geographic areas into two localised parallel dimensions occupying the same geographic space. Once “split,” changes in one localised parallel dimension do not affect events that go on in the other.* However, space outside the area in which the Reality Splitter is used is entirely unaffected. When people enter a physical area in which the Reality Splitter was used for the first time, they must choose up to one of the parallel dimensions to enter. Once that person chooses which dimension to enter, they will only ever “see” that dimension when they occupy that physical area for the rest of their lives.
*For example, if someone were to use the Reality Splitter in a forest, two forests occupying the same overlapping space would now exist. In one dimension, people could remove trees from the forest, while in the other dimension the forest would be entirely unaffected.