I get this question a lot. It is stated in the license agreement ( http://www.pokertracker3.com/products/PTH/download.php ) that you can register Poker Tracker on 2 different computers (YOUR OWN COMPUTERS THAT IS - you cannot share codes) with the purchase of one license.
Each computer will have a different product ID (located on the Help/About Poker Tracker window) so you will need to get a different reg. code for each computer. When you buy PT, you should send an email with the product ID's you want to have registered and I'll send you the 2 codes.
NOTE: If you don't have the program installed on the second PC when you purchase Poker Tracker then you can email me the product ID from that computer at any time later and I'll send a code for it. You are not required to send both product ID's at the time of purchase.