This is a great question; it was originally asked a few months before I signed up, so I couldn’t provide an answer when it was first put out on the boards, but I’m glad I can give an answer to it now.
When we do programming (we do a great deal of it for the web) we need to have several different windows installations on several of our machines (so multiple windows installs, but on many different computers). On top of that, many of our workstations had multiple installations of operating systems on them already even before they were turned into workstations. For example, one of the computers I was using this morning has both windows 7 and 8 on it, and now also has windows 10 installed on it as well because we need to test a lot of the JQuery code we write on the “edge” browser (which is only in windows 10). So on that machine there is a total of three.
Here’s how we do it; what you have to do is create several different partitions on a single drive. So, on the machine I was referring to above, there are a total of seven partitions. Three of them are about 100 GB and one has windows 7 on it, one has 8, and one 10. That’s exactly the size you need by the way for such an installation in terms of gigabytes. And also that’s basically how you put multiple versions of windows on one machine. You have to create several partitions, insert the install media for the specific operating system, boot to it, and install the version of windows you want on the empty drive. The boot loader on your main windows installation should be able to handle on startup allowing you to select the operating system you want when your computer boots. And any version of windows will do that automatically for you in the first place, so you don’t have to worry. Three of the other partitions handle program files for each of the respective operating systems, and they have a size of about 20 gigabytes each. The last partition is what we call a “stuff” drive and it’s about 2 TBs and it’s simply where everything is kept.
Hope that answers your question.