I plan to do a pc upgrade very soon. Alongside that I plan to start with Mint or Fedora. Is there any real or big downsides to dual booting ? Aside from the harddrive space lost ?
If I like or really like my time with Mint I would probably switch permanently but I felt like I wanted to test it for a couple of months before making a complete switch.
Im a big 
so that impacts my consideration. Dont really play much multiplayer shooters so I dont have a problem with kernel anticheat games not working.
I have done what you’re talking about doing for 25 years.
There is no need to use separate disks for each os. That’s just complicating things.
Before you start, make a backup, make sure bitlocker and quick startup are turned off in windows then go for it.
Afterwards, practice reinstalling grub (your linux bootloader) from a usb environment. That will be your lifeboat process if windows decides to replace your bootloader.
*when windows decides to replace your bootloader.
Let’s be serious heee, there’s a good chance the op goes back to just windows before that can happen.