Your pro lasts for 6 months so if you purchased pro less than 6 months ago then you can update. You can run it on as many as you like. You can downld it separately onto the 2nd comp or else copy it. After downlding it will be a dmg file. So just copy it across via zip disk or cd or network, etc. After your pro registration you will have received an email with a link to your personal downld page. You should bookmark this. If you no longer have the email then try Here (click on link)