Having two drives is sometimes not enough, either. I have no idea why, but anytime Windows installs for the first time or goes through a major update (not the small security patches, but the periodic feature releases) there's a random D20 dice throw to determine if it will randomly decide to create the bootloader and recovery partitions in another drive, even though your main installation isn't there.
I kid you not, Windows 10 once decided that my external SSD enclosure was the best place to put the bootloader.