Yes, this should work with no problem. After installing sabayon on your new drive, power down and plug your old drive in. It's not really necessary to do it that way, but if you are uncertain about how to install a dual boot, it is a good practice until you know how to install a dual boot without the extra trouble of resetting the bios hard drive boot order every time you want to boot a different OS.
Before you reinstall your old hard drive, it might be a good idea to practice installing sabayon dual boot with itself. In other words, install sabayon on a separate partition, then install again on another partition. Learning how to install using the "custom partition and install" option, allows you to take some risks with your installation, knowing that you can reinstall or use an alternative installation. Just remember to unplug your windows drive before any sabayon installs. By the way, I've done what you are suggesting many times when I was learning linux.