Great question — answering as the UphillTech team:
Install Windows on a SSD/HDD. Encrypt it completely. Disconnect it. Install Linux on another SSD/HDD. Encrypt it completely. Reconnect the Windows one also. Use BIOS/UEFI quick boot select key to select which one you want to boot into. This way, both systems are completely encrypted and do not interfere with each other.
Full answer by Overmind: https://superuser.stackexchange.com/a/1530446 (CC BY-SA 4.0).
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