How do you configure multiple systemd services to use one timer?

If you want to activate multiple services with a single timer insert a target in between:

The timer unit, let's call it foo.timer:

[Unit]
Description=My timer that runs saturdays, 9am and triggers foo.target

[Timer]
OnCalendar=Sat 9:00
Unit=foo.target

[Install]
WantedBy=timers.target

The target unit, let's call it foo.target:

[Unit]
Description= My target unit, that groups my two services xxx.service and yyy.service
Wants=xxx.service yyy.service
After=xxx.service yyy.service

[Install]
Also=foo.timer

And then the two services xxx.service and yyy.service:

[Unit]
Description=My service XXX

[Service]
ExecStart=/bin/echo I am XXX

[Install]
Also=foo.timer

[Unit]
Descritpion=My service YYY

[Service]
ExecStart=/bin/echo I am YYYY

[Install]
Also=foo.timer

Copy these four unit files (foo.timer, foo.target, xxx.service, yyy.service) into /etc/systemd/systemd/. Then enable and start the timer by issuing "systemctl enable --now foo.timer". That will hook foo.timer into timers.target, which is the generic target that is supposed to pull in all the various timers defined on a system. Note that you could as well do "systemctl enable foo.target" btw, and also "systemctl enable zzz.service", since the Also= lines in those units propagate the enablement requests towards foo.timer.