The Vesica Piscis is a great example of my worldview. Imagine that the circle on the right is the physical world of our reality, and that the circle on the left is the world of spirit (otherside, otherworld) that some can travel into.
The area where they overlap is the "Underworld", or Elphame. The realm where there is some semblance of this world, either in geological features, structures, or perception of the physical, while still being paired with imagery of the spirit realm.
Myself, I also count the borders. The Hedge is the border. In the illustration, it is the black line that surrounds each circle.
It was impressed upon me that borders are areas that simultaneously exist and don't exist. While you generally think of a wall as a two dimensional surface, an entire ecosystem can thrive between two bits of drywall. Such is the Hedge, for me at least. Just as it can be a boundary marker, so can it be a strangely dappled world where things live and thrive.
One author (oddly enough, an author of fanfiction) described something like this as an interstitial dimension. Depending on where, and how, the border is approached, it can be a very different experience.
In addition, consider that the two circles may not be the only ones. Multiple spheres and worlds can just as easily be chained along existence (one upon the other) meaning that the borders of multiple worlds overlap.
Some plants I speak to beneath "our" Hedge. In this border, everything is very much like our world, only a bit louder, so to speak. Others, I deal with beneath the Hedge beyond the Underworld, but only the plants that I trust. The images we deal with, even in the Underworld are nothing compared to how they appear beneath that hedge. Or beneath the hedge where the Underworld meets with something far more distant.