Vista 64 questions

Are you trying to install Vista 64 on a different hard disk or a different partition?

Not unless you have a separate license for each operating system.

If you're trying to use the same license product key thats illegal and MSFT seriously frowns on that!

And if somehow you successfully pull it off using only one license and MSFT discovers it, they will kill your product key, and you won't run Vista anything with that key anymore, so be careful how you proceed.

Now if you do have 2 separate license keys, just unhook the drive the OP/SYS is already installed on, hook up the new HDD to install the second OP/SYS to it, delete the present partition information on it and create a new partition for the new installation and proceed with the installation, allow the installation to fully complete and install all the necessary drivers, then hookup the other HDD, and while booting hold down F8 to access the boot manager to choose which OP/SYS to boot into.