When you purchase Astro Gold macOS, or, Astro Gold Windows, you purchase a license, which is actually a dual license. What this means is that once you have that license, you can use it to register both apps.
So if you are on your Mac computer, and download the Demo of Astro Gold macOS, you can use the license to register it. If you then go to your Windows computer, and download the Demo of Astro Gold Windows, you can use the SAME license to register that app. Or Visa Versa.
If you already own a license for Astro Gold macOS, there’s nothing extra to purchase. Your existing licence details can now also be used to register Astro Gold for Windows. Likewise, anyone purchasing Astro Gold Windows will also be able to register Astro Gold macOS with their license details.