Yes, you can activate two copies of Nitro Pro on two different machines with the same serial.
Below is an extract from Nitro Pro's End User License Agreement (EULA): __________________________________________________________
You may install one copy of the Licensed Software on a single computer.
The primary user of the computer on which the Licensed Software is installed may install a second copy of the software for his or her exclusive use on either a portable computer or a computer located at his or her home, provided the software on the portable or home computer is not used at the same time as the software on the primary computer.
If Licensed Software includes multiple licenses, Licensee may install the number of copies licensed to Licensee by Nitro PDF Software.