This is VERY easy, though I've never done it with Windows 7 yet and I've never tried installing the same OS twice. I really don't see why you'd want to have 2 Win 7 installs, but that's besides the point.
When installing Windows 7, at the beginning at some point you'll get the option to configure the Hard Drive. If there is an option to go Advanced, Do So. Then if it's 1 Hard Drive you're doing this to: Format, Then Create a second Partition from the Free Space you have, make sure you give each partition enough space. You should now have 2 Partitions (maybe 3 sometimes Windows 7 creates a small partition for system shit), Now proceed to install Win 7 on the First Partition you made. After you complete the OS Install and are ready to install the next OS, just boot from the OS CD/DVD again. You'll come to the Configure Hard Drive screen, now select the Second Partition you made and Install. That Easy, I Usually have a Dual Boot of Windows and Linux.
Booting from a USB as a Dual Boot never works well (For me anyway), though I'd be curious to see how it would work with USB 3.
EDIT:
Now, if you have a Windows 7 already installed and want to keep it, you would need to make sure you have enough free space for another Install. Then you need to split the free space into another partition, you can do this by using the Windows CD/DVD Hard Drive Configuration (as above, there will be a resize button) or you can use a program called Partition Magic (Good Program, not free... legally). You can try doing it in Disc Management within Windows, but since you're doing it to the Hard Drive that you're running windows on, it'll probably fight you.
Make sure you give yourself enough free space for both Windows Installs.