Whoa there, wrong information. Booting with 2 system drives installed will simply create a multi-boot situation. You will still be able to boot with either one connected just fine, but you will have the multi-boot files remaining on the "active" drive. You can't have 2 "active drives" at the same time under any circumstances, Windows will not allow it and will set only one as active. Most times Windows will handle these scenarios alright, but sometimes a utility is needed to set "Active" volumes for booting.
It's still good advice to avoid booting with 2 system drives connected, but it's not the end of the world if you do.