You're allowed to activate it on two at a time, but only allowed to run one at a time.
As Jim points out, the Activation is the important aspect, as one could have more than two un-Activated copies, but obviously, one would not be able to use an un-Activated copy. Some users have come up with involved schemes to De-Activate various Adobe programs, for ultimate security (cannot see this being useful, but many seem to), but then one hits another issue - Adobe programs allow X number of installs, and Y number of Activations. While Adobe Customer Support can reset those numbers, I do not think they would want to have to do that about each month.
So, Jim's answer is correct - 2 Activated copies, but not run at the same time.