I don’t think you can take one primitive and add it to two viewers like that. There’s a lot of assumptions about the state of the primitive that won’t be compatible with this setup.
You might need to keep track of the global state yourself and synchronize the two primitives.
I think a better approach is if CesiumJS could somehow support a multi-camera view mode, that would be in the same viewer, kind of like how the imagery split works:
Can you describe more about your use case? That’ll help us collect information for figuring out if this is a feature that’ll be useful to users/the community.