Can you have one copy of Photoshop on two devices?

I have been told by a friend that it is possible to have one copy of photoshop on two devices, however i don't know if this is true or not. I installed my copy of photoshop onto my imac around this time last year, but i have recently bought a macook pro and i would like to download it onto this computer. I would like to know if this is possible and if it is how do i do it? I have no disc drive on my macbook pro so this could also be another issue.

Thanks