It is no problem to use an Apple Bluetooth Magic Keyboard with a Mac and an Windows computer. The following steps should be taken into account:
1. If the Magic Keyboard was first paired with the Mac, you have to unpair it before you pair it with Windows computer.
2. Afterwards pair the keyboard again with your Mac.
3. Obviously 🙂 Disable Bluetooth on the computer you’re not using. if you’re using them side by side.