As I understand it, it can be activated on two computers at a time, but the two copies of the software cannot be in use a the same time.
As SRiegel stated that it can be active on two computer, you can actually have it installed on multiple computers. When you try and start PS, it will tell you that is is installed on other computers and ask if you want to sign off the other computers. You just say yes and sign in to the one you're working on. It's very easy, and that way you can have various computers to use and sync all you settings to those computers.