try this
- turn off pc
- disconnect the drive you don't want to boot from
- after everything is up and running, reconnect the drive
if you can read the drive upon reconnecting, my guess is there is some kind of safety feature that keeps the system from using two OS drives (with same OS, so two Win 10s rather than Win 10 and Linux) at once.
There is another issue. Win 10 is theoretically portable. Say your mobo crashes and you replace it. You can use your Win 10 and product key to work on the new motherboard. (You may need to install the OS again, though)
If on the other hand you have a working computer with a Win 10 install and you try to build a second computer with the same (same product key) Win 10 install, the OS is not going to work on the older install. One at a time is the rule.
If you want to have two Win 10 OS drives active with the same product key you might meet with a refusal on the part of the active system to work with both at once. Which ever one you use is the "new" one and the unused one becomes the "old one" that will get shut out.
You could test this by getting a second version of windows and see how it works.