When using the prepass renderer, multi-viewport rendering doesn’t behave the same way as without it.
Without prepass:
With prepass:
It appears that each view is rendered to a full-frame RT when the prepass renderer isn’t active. However, when the prepass renderer is enabled, the viewport of the camera is applied both while rendering the view to the RT and during post-processing.
We do not support generating geometry buffers for multiple camera views… It’s not supported by the pre-pass renderer, but it’s not supported by the default geometry buffer renderer either.
However, you can create instances of the geometry buffer renderer yourself (not through scene.enableGeometryBufferRenderer()), and create one instance for camera1 and another for camera2: