Normal bubbles in our 5th wall don't merge with each other, but if a Portal between 2 5th walls opens, two beyond bubbles could merge with each other, leading to catastrophe, such as two creators and two destroyers, which would cause unfathomable amounts of chaos. The Aspect Lords are still outside of The Unimaginable, the area where they reside is called the 6th wall, where they oversee multiple 5th walls throughout The Box.