You can use Luna and Astropad by going into the Astropad application and then hold down the circle button. From there you should be able to see a bar in the center, if you were to change that you Luna, you can utilize Luna + Astropad together.
Luna can work over USB for better performance if you wish! All you have to do is connect your iPad to your Mac directly with a USB cable. The dongle is there so that your Mac thinks you’ve plugged in an external display. By doing so, we can utilize the hardware acceleration capabilities.
You can get Astropad to show not both screens per se, but a section of what you see on your Mac Display. If you hold down the circle button, you can resize and move the area of which is displays to your preference. Alternatively, if you’re using Luna within Astropad, you can go into your System Preferences -> Display -> Arrangement -> check mirror displays.
I hope this helps!