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#1
Welcome to the forum the simplest way is to use a free boot loader there are a few of them Free Boot Managers and Multi-Boot Loaders (thefreecountry.com)
Better yet, just set a default OS in the Bios, then use the one time boot menu to boot into the other OS when you want to, hopefully you installed each OS while the other drive was unplugged.
But you can try EasyBCD - NeoSmart Technologies