Activate each computer
One license key works on 2 computers. Activate it online on each computer. To use a third computer, free a slot with Move my license first.
Move or sync the company file
Move your .fqb company file between computers, for example on a USB drive or through a sync folder. You may keep it in iCloud Drive or Dropbox, but open the company on one computer at a time. Keep backups as well.
Never have the same company file open on both computers at once.