Hi
You must install different windows versions to different hard drive partitions
The booted windows partition will be c: when windows active
It is likely the boot manager will be on the exiting hard disk so if this failed the other hard drive would not be bootable until a emergency repair has been made
To keep every thing separate you need to remove existing hard drive and install windows on remaining hard drive
Then refit old hard drive
Actually no need to physically remove hard drive only disconnect sata data & power cables
You would also need a bios which lets you choose boot hard drive at bios startup
Often F12 or F8
Regards
Mike Barnes