Hi there,
Without being silly about it, you can install and run on as many PCs as you want. I have 15+ PCs and have Trainz etc installed on most. However, I only play on one PC at any given time. It is good to have more than one installs as I find often some of my PCs are doing other jobs, such as imaging hard disk etc. PTP 2 should install on a 2nd PC without any problems.
Having said all that, I support Auran / PTP and buy each version of Trainz & / or ad-ons multiple times. I very much appreciate their excellent policy on software install and use.
Regards, Mimes