Here is a special device that fits into a 5.25 hard drive bay.
https://www.startech.com/HDD/Mobile-Racks/hot-swap-hard-drive-bay-usb-enclosure~S251BU31REM#
It enables you to swap SSD's by sliding them into place in a drawer. A Sata iii connection is made when the drawer is pushed all the way home.
The product also offers a USB 3.0 link, but I am interested in the SATA iii connection. In effect, it makes the SSD you slide into place an "internal drive," even though it is easily swapped for another SSD.
The idea is to create two distinct SSD's, one with Win 10 installed, the other with Win 7 installed.
To switch operating systems from Win 7 to Win 10, for example, you would shut down the computer, swap the Win 7 SSD for the Win 10 SSD, and then reboot.
Would this work?
Thank you for your insights, John