1) Use your motherboard's boot selection menu, by pressing a function key, at boot time to choose which drive to boot to.
Pros: You can wipe a drive and have no impact on the other.
You can have the active OS always on drive C.
Cons: You will need to use the boot menu at each boot if your default setup is not what you want. Mind you, you can change the default in the BIOS.
You cannot do this if you have a very old motherboard that does not offer accessing the boot menu by pressing a function key.
2) Install a newer OS after an old OS.
Pro: You will get a boot menu offered by your new OS.
Cons: You will lose the menu if something gets messed up with the new OS, or if you decide to remove the new OS.
Your new OS will be assigned drive letter D. When you install a program, by default, it will install on drive C. Things can get messy very fast if you do not pay attention when you install new programs/games.