It depends on the date of registration of your serial numbers. If you register both numbers on the same day, the license periods will start and expire in three months. If you register one serial number and register the second one three months later, you will be using licensed Dr.Web software for six months. But this may not be a good idea since you won't be able to get a renewal discount for two three-month key files, and two short-term licenses will cost you more than one six-month license.