Yes.
From pool's home page:
"Every instance of your miner needs its own worker account!!!"
You could have also seen that your grand total hash rate was staying the same running the two PCs as the same worker, compared to only running one PC. I believe running two PCs as the same worker splits the work in half, so you were getting the same reward as you would have with just one PC. (I believe).
http://mining.bitcoin.cz/accounts/profile/worker-edit/Anyway, register a different worker and you're fine. Edit the batch script on one of the PCs so it uses a second worker.
Question: Does it reduce my reward if I run 2 instances of a mining program on 2 different pc's with the same worker account?
The reason I do it is: I use the same batch script, and I currently don't know how to log into a different account (since the pc's are completely identical)