I think you will have to designate one of the drives as the default boot drive with the other win 10 as a boot menu option. Try installing EasyBCD on the boot drive.
You only need EasyBCD once.
It's not involved in the actual boot process, it just manages the contents of your BCD.
Choose one of your W10's as the default, put EasyBCD on there, and use it to add an entry for the other OS
EasyBCD is a powerful bootloader-modification tool that has a wide-range of applications and usages. It can perform a variety of tasks ranging in complexity and application from the most basic to a…