A Lightning Payroll subscription allows you to download the desktop version of the program onto two separate computers. However, the two installations do not talk to each other automatically, and do not allow simultaneous use - keeping in mind that the Lightning Payroll desktop application stores data locally, on your machine, and not online.
Note that we now have the option of the web/online version of Lightning Payroll, as well as mobile app for Android and iOS available to subscribers, and these options are not limited to any number of devices, and do not require backing up, as the data is live and online.