Sync Mode may also be used to make changes to translation for two branches simultaneously. Set Branch folder path in your project options to the path that corresponds to base folder of the branch, and Secondary Sync view will automatically open files from branch. Then, each time you make changes in files of your main branch, they will automatically be replicated to the branch (of course, if it contains the same English string).

For example, if you work on KDE translation, you can checkout trunk to /home/xx/hacking/kde/trunk/l10n-kf5/YOUR_LANG and branch to /home/xx/hacking/kde/branches/stable/l10n-kf5/YOUR_LANG. Create Lokalize project: /home/xx/hacking/kde/trunk/l10n-kf5/YOUR_LANG/project.lokalize and set BranchDir=../../../branches/stable/l10n-kf5/YOUR_LANG, then work via this project, and commit changes in both trunk and branch folders.