Hmm, Hard disk to Hard disk?
Well you see, here's the problem with doing that, If you start the setup ISO from HDD 1 Assuming its your current windows drive, And ask it to install windows on HDD 2, It will not work if you plan to separate them eventually.
Because if you ask the setup to install windows on HDD 2, It is the boot sector of HDD 1 that will be written not HDD 2, So if you plan to separate them, HDD 2's OS will stop working because its records are on HDD 1.
So if you want to keep the disks connected to each other it can work, if you want to take HDD2 somewhere else, HDD 2's OS will not work on its own unless installed again ON HDD 2.
Hope my jibber jabber helped.