There are many people (even though the ones that you @ed in another thread are no longer active here). The difficulty with answering this kind of question is that the answer will always start with “It depends…”, and then can quickly derail into deep, technical considerations about the requirements, project setup, goals, and pros and cons of different approaches.
Generally speaking, it does not seem to be possible to have “one scene” that is shown in 2D and 3D simultaneously.
(I think that this would be nice to have. And when drawing a high-level diagram about some renderer architecture, it wouldn’t be clear why this should not be possible. But … the devil is in the detail).
In another thread that you opened, you already seem to describe what could be the basic approach for solving this - namely, to create two Viewer objects, one for 2D and one for 3D, and then just add everything that should be rendered to both viewers.
Depending on what exactly is added there (and how), this may require some care and special considerations. For example, something like
const model = loadSomeModel();
viewerA.scene.primitives.add(model);
viewerB.scene.primitives.add(model);
might or might not work (and I’d guess that it does not work). Instead, you may have to load the model twice.
So … when you encounter a specific problem with this approach, it would be best to carve out a minimal example that shows the problem, and post it here as a Sandcastle. This will allow others to provide more focussed help more quickly.