Unfortunately I don't know of a way to do this. Powercfg only lets you change the same settings you see in the Power options in Control Panel. Perhaps disabling the other HDD completely via Device Manager will cause it to spin down. That could possible be scripted - though Windows sometimes requires a restart when you disable/enable some hardware.
In terms of temps, it wont make much difference, especially if another HDD is always spinning. Plus the power savings are absolutely negligible.
In terms of temps, it wont make much difference, especially if another HDD is always spinning. Plus the power savings are absolutely negligible.