You can install it on multiple systems, for sure, and play it from either of them as well... Once again I'm assuming this based on what I've seen from other games. You will not, however, be able to log in under one account from 2 locations simultaneously. This would allow some users to multi-assist and consequently ruin the equality of the game for everyone else.

Of course.... one could just as easily get 2 accounts and lock a WMs cure spell action on his Warrior... If one were willing to spend that kind of money just to get an easy ride.