Opacity mask(?) following the camera that hides parts of a large textured object but not another textured object

Difficult to come up with a title for this one that makes sense.

I’m working in VR with a big inverted sphere for a background, and using materials on a plane in-front of the camera which applies blurs and other changes to the view by using translucent materials. All working fine.

The next thing I’d like to do is to have 2 “worlds” that break through onto each other. Imagine it as 2 spheres with 2 different textures that sit in the same world space, but wherever the viewer is looking, they see the 2nd world, but all around them is the first. (Image as an example).

I’ve got some ideas around using an opacity mask on the space texture that moves based on camera position and putting that object slightly closer so it always occludes the castle, but was wandering if there’s a different way?