Steam's in-home streaming takes up both devices. When you're moving the mouse and pressing keys on your client (the underpowered laptop), it's sending those same key presses and mouse movents to the host (your gaming PC). Computers don't really respond well/aren't designed to have multiple users in the same desktop environment at the same time.

Another user linked Wolf which uses Docker containers, or you'd need to set up a virtual machine inside your gaming PC that would become the host instead.

Is it possible? Sure.

Is it viable? No.

This is exactly what Wolf is meant for. It works great!

I would say: Keep it simple and use Steam's in-home-streaming. Both computers will be busy playing the game (the beefy computer rending and streaming to the thin client). So it doesn't do exactly what you want, but it does let you stream easily.

Do you want to still use your laptop for other stuff?

Doesn’t Steam have the SteamLink stuff open sourced? Can you just install that and make the laptop a SteamLink client? I think people do it with Raspberry Pis all the time

Maybe if your cpu has graphics, you could run a VM and pass your dGPU to it and streams from inside the VM.

Otherwise I think the inputs will fight each other, even with virtual displays (one person using the pc and the other gaming via stream)