At the same time, no. You can have two accounts logged into at the same time and applications running in each, but only one account is accessable at a time. The other account will continue to run in the background.
You need Windows multipoint server 2011. You will need 2 server licenses, and 2 client licenses to make it legal. PM me if you need more info. I think it works great and saved me lots of $$ from buying multiple PCs.