I have Windows 7, 8.1 and 10 installed on different SSD's and can boot to any which one I like depending on what I need to do. If someone I support has a Windows 7 issue, I can boot Windows 7 to emulate what they are doing, help them through troubleshooting steps, etc. and it works great. I had a lot of issues with booting early on because I had all 3 drives attached and the first install would go great, but then the second would cause my first one to not boot... So what I recommend is when you are installing that you only have the SSD attached that you are installing to. After you have them both done, then you can have both connected at the same time and just use the computers boot menu to pick the drive to boot to (and thus the OS you want to use). There may be other fancier ways of doing it but this one works just great. And with the boot speed of SSD's it is only a matter of seconds to restart and choose a new OS...