I'll try and answer all your questions one by one. I'm assuming you have a Win8 Setup DVD/USB handy, which you created using the Upgrade Assistant.
First of all, the easiest solution by far would be to attach only the 2TB Win7 drive, boot from the Win8 Setup media, shrink the Win7 partition (if it's using all 2TB) and install Win8. No mess, no fuss, and you'll end up with the Win8 graphical bootloader that will allow you to select the OS you want to boot:
If you are dead set on copying the Win8 version you already have installed on the 500GB drive, attach both drives and set the 2TB Win7 one as the first boot device in the BIOS. Once you're in Win7, install EaseUS Partition Master Free Edition and shrink the Win7 partition till you have enough unallocated space to fit the Win8 partition:
Now copy the Win8 partition's contents to the newly created unallocated space:
Next, install EasyBCD and you can add the newly copied Win8 to the Win7 boot menu:
After disconnecting the 500GB drive you should see the Win7 text-based boot menu with both OSes listed:
If you prefer the Win8 graphical bootloader you can run Automatic Repair from the Setup media:
This should do the trick and allow you to use the graphical bootloader to boot into either Win8 or Win7. However, in case the graphical menu does not list Win7 for some reason you can always install EasyBCD in Win8 and add an entry for Win7.
Didn't I tell you reinstalling Win8 afresh would be the easiest way? :)
One more important detail. Since you seem to have clean installed Win8 Upgrade on an empty drive, it's quite likely you will not be able to activate successfully (the Upgrade version requires a qualifying older version of the OS to be already installed). If you face this issue see here for a couple of workarounds. As with all activation-related problems, last resort if nothing works is to call MS and use their automated system or talk to a Support Rep to get the issue resolved.
Finally, coming to the license transfer. Win8 Upgrade replaces your older OS' license (Vista in your case), so you can no longer use that Vista key again as long as you're using the Win8 Upgrade version. You can of course transfer the Upgrade version to another system as long as you don't have multiple copies using the same key at any given point of time.