The knock spell says (emphasis ine):
A target that is held shut by a mundane lock or that is stuck or barred becomes unlocked, unstuck, or unbarred. If the object has multiple locks, only one of them is unlocked.
If you choose a target that is held shut with arcane lock, that spell is suppressed for 10 minutes, during which time the target can be opened and shut normally.
When read separately, it is clear that knock only unlocks one lock each time, and knock suppresses arcane lock cast on the target.
However, what happens if the target has both a mundane lock and is locked with arcane lock?
- Do both of them get unlocked?
- Or perhaps because arcane lock counts as a lock, does it only unlock either the mundane or the arcane lock (not both)?