Well, apparently you have to be really careful which one you install first and even then there can be problems, as someone once told me in another forum:
"Yes, the Windows partition will be treated as its own drive, and the GNU/Linux distro install will be treated as its own drive. HOWEVER, please be aware that the two OSes can in some cases interact with each other. For example Microsoft has at some occasions released update to Windows 10 which breaks GRUB. One example of this was the Creator's update which for seemingly no reason created a hidden partition after the first partition (thus changing the partition table on the HDD, which creates a mismatch between it and where GRUB looks for files)."
Here's the rest of my thread for context when I was trying to see if I could run Linux for older games and as a main OS.
Also, keep in mind, if Windows 7 is working for someone (me, in this case) offline, then Linux has no purpose for being used and could just be a hassle with people's hardware and peripherals not being compatible. Then, there's the annoyance of finding a distro that is the best combination of community supported and easy enough for beginners.