Is it possible to have it run at multiple moments by declaring it in the same timer?
Sim.
Veja este trecho de man systemd.timer
(minha ênfase):
OnCalendar= Defines realtime (i.e. wallclock) timers with calendar event expressions. See systemd.time(7) for more information on the syntax of calendar event expressions. Otherwise, the semantics are similar to OnActiveSec= and related settings. Note that timers do not necessarily expire at the precise time configured with this setting, as it is subject to the AccuracySec= setting below. May be specified more than once.
Aqui está um exemplo de trabalho que uso:
[Timer]
OnCalendar=Mon-Sun *-*-* 23:00:00
OnCalendar=Mon-Sun *-*-* 06:00:00