Question about ASUS ROG Zephyrus Duo 15

Hmm, that's right, the small screen is fixed to the opening of the main screen. It opens as soon as you open it. But there are no air inlets or outlets underneath. The speakers are underneath. The cooling performance should be a little muffled, but not significantly. I tested it briefly, Shadow of the Tomb Rider/Daying Light open and closed. The temperature of the CPU/GPU hardly changes at all.
However, closing the screen does not remove the screen from the devices as it does on other laptops, and that takes a bit of getting used to. Of course, you can switch it off manually.
If you plan to use it with the screen closed, then I would choose a different product, as a significant part of the price is driven by the fact that a second screen is built in ... which is cool :)

The position of the small screen is fixed. And yes, the opening is mandatory for the power.