Two options...
1) Create a new Partition on your HDD\SSD for Win7.
- Install Win7 on another HDD\SSD, but this method requires a reboot into BIOS to swap between OS's. Having Win7 on a secondary drive
does get around a few issues that come with having two OS partitions on one drive though.
- Note that you also need a valid activation key, so it's not free (unless you never activate, which technically = pirate)
"I figured that it would be nice to have 2 windows versions"
- Unless you have a specific program (not all that common) that will not run on Win10, dual booting is pretty much pointless.
I just removed the drive I had Win7 (I upgraded from 7>10) on for the past six years because, even though I did need it for one thing
(Nvidia 3D Vision didn't work with a few games on Win10, but did with others), I ended up never using it after the first six months
when I also used it to compare gaming performance with Win10 (insignificant difference at that time, btw).
Also note that MS have stopped doing security updates for Win7, so if it becomes infected by a virus\malware\ransomeware
your other drives may become infected as well.
- Actually, if you want to experiment with different OS's it would probably be better to use a Linux distro.
They are free, and use fewer system resources than Windows. Programs and games are more limited than Windows,
but there are also many free programs for Linux
[Many of which work on Windows as well, eg- The Gimp (photo editing) Blender (3D modeling), OpenOffice.org and more].