Answer
A Lightning Payroll subscription lets you install the desktop version on two separate computers. The two installations do not talk to each other automatically and cannot be used at the same time, because the desktop application stores its data locally on each machine rather than online.
If you need to work across more devices, the web/online version of Lightning Payroll and the mobile app for Android and iOS are also available to subscribers. These are not limited to any number of devices and do not require backing up, because the data is live and online.