As I recall, PARK would park the drive heads PRIOR to spin down, but that was always controlled by power off.
Power Management in XP can power down drives, but not individually.
What you need is a switch. But, there is always a risk of powering down a drive (IDE) while the computer is running. If you installed your two drives in mobile racks, they have independent on/off switches, but I would be very wary of turning one off in mid-stream.