If you really have 2 licences (it means, you didn’t upgrade, but you bought a new licences), you have to buy a new USB-eLicenser, then connect both to the same computer, and drag-and-drop one of the licence to the new USB-eLicenser, in the eLicenser Control Center application.
Why do you need two computers with VST System Link?
When you upgraded, it means, you have only one licence. Your Cubase 5 licence was upgraded to Cubase 8. You can use Cubase 5 with your Cubase 8 licence. But, you don’t have 2 licences, and you cannot run Cubase 5 on one system, and Cubase 8 on another one system.
If you want to use 2 computers, I would strongly recommend you to have a look on Vienna Ensemble Pro.
For the licences, I don’t believe it’s fair. The cubase 5 → 8 update costed already 300€ ! Even more expensive than many non update (entirely new) DAW ! I hesitated a lot before to buy : I saw Steiberg asking 100 or 200€ each year !
With all the money I left to Steinberg, they could, at least, let the cubase 5 licence for people like me who wants to use several pc. Cubase 5 would just become a vst host, that’s it ! And Steinberg doesn’t offer any vst host like vstack did.
Anyway, if they don’t want (I asked them & wait for the answer), I’ll never give them any money for Cubase 5 licence and I’ll buy Vienna pro.
But in fact, this is normal for any software. If you upgrade your license, you still have one license only. You don’t buy a new license, by the upgrade.