>> "Can that key be used on two different Pc's activating once each. ie a two pc license?"
Yes.
>> "If so why does the user say it doesn't work as the second Pc reverts to trial expired and asks for another key after working for a day? "
My guess:
1. The customer is activating on multiple computers.
Or
2. The customer is activating on a VM that's moving between computers.
Or
3. The customer is modifying their computer.
You need to tell us the product key so we can tell you exactly what has changed. Also, the way to solve VMs changing is to use TurboFloat.