One part-timer works at both my branches. Do I add her twice?

Yes. Each branch is its own restaurant here, with its own list under Settings → The POS → People at the POS, so add her at each branch with a PIN. The PIN can be the same four numbers at both if that is easier for her. Her hours are then recorded at the branch where she clocked in, and Reports → Hours worked at each branch shows its own stretches. If your branches are grouped, the reports page across your branches has an Hours worked, every branch panel that shows who was on, at which branch and for how long.