Sharing effects between windows doesn't work. If tried, the entire View3D content is not drawn on the other window. For example, see the attached application.
The following trace can be seen in output:
The reason seems to be that Quick3D caches the shader data into current renderContext at initial QQuick3DEffect::updateSpatialNode call, so it won't be available on other contexts.
This blocks showing effect previews in QDS.