Comment 2 for bug 1870144

Installing chrony with timesyncd running (the default) is what triggers the "both running" situation.
I have fixed this in the past, something must have changed to open this issue again.

A restart on systemd-timesyncd would detect and stop it.

Must be due to:
  45 chrony (3.5-6) unstable; urgency=medium
  46
  47 * debian/chrony.service:
  48 - Don’t conflict with systemd-timesyncd.service.
  49 A few users complain that chronyd does not start at boot. The way the
  50 Conflict= directive works internally might cause both systemd-timesyncd
  51 and chronyd to be inactive at boot. So by relying solely on the
  52 disable-with-time-daemon.conf drop-in file provided by systemd, we should
  53 get rid of this malfunction while still preventing these two time daemons
  54 from being active at the same time. Kudos notably go to Santiago Vila for
  55 the report and providing SSH access to a GCE instance where the issue was
  56 reproducible and Michael Biebl for debugging. (Closes: #947936)