When ever I want to dual boot operating systems I go for separate HDD's.
What I did.
Get hold of a cheap second hand drive or a small cheap new one if you want.
With the power to the PC turned off, unplug the system HDD that is installed.
From a different connector install the new drive and power up the system.
Install the "guest OS" just like it was going to be your only OS.
Once installed and set up power down the PC again.
Reconnect the original HDD so you now have 2 drives on different connectors.
Power up the PC and using the bios select which drive to boot from.
Mine is F12 during start up to get the selection menu.
Check with your motherboard manual for yours.
It might sound like a pain but it anything goes wrong you cant mess up your main drive.