In the Menu action for the Night Auto Attendant:
On the Touch Tones tab, check the box for Timeout, set the time to 2 seconds.
Connect the Timeout to a "Increment and Test Counter" action, set the "Value to test counter against" to 2
Connect the False back to the Menu action for the Night Auto Attendant
Connect the True to a Leave Mail action for mailbox 600

Call comes in, hits the Night Auto Attendant. Caller doesn't dial anything, it times out, the counter increments to 1, it is false, the call goes back to the Night Auto Attendant. This time when it times out, the counter increments to 2, it is now True, and goes to mailbox 600.

I would also suggest just making option 0 go to Mailbox 600, so the recording can say "to leave a message in our general mailbox, press 0", and the caller can get there faster. If they can't follow directions, twice, they get there anyway