I tried to use TWS on different PCs, but I got an error message telling I cannot use it on several devices. Can't I use it on two different PCs?
This depends entirely on your subscription package and based on the limitations provided or imposed by that subscription. Basically, I am trying to say that you have to check with the ones you've acquired the subscription from and ask them about your inquiry. There is always the possibility that you won't be allowed to use one account on two PCs.
I recommend contacting support through the following link that will lead you to the official page: https://www.geoworkz.com/Support/Default.aspx