It doesn't happen very often but it does get annoying when you are stuck behind someone who can't get the second gate to open.
I prefer the set up in SYD, where the first door is actually a "check-in" machine where you obtain a ticket - spread out along the walkway towards immigration, then queue up and go to the first free face-recognition machine. What I can't remember is whether there is a single queue or each face-recogniser has a separate queue. Also SYD uses a lot of paper. A reusable RFID card and electronic declarations might be better.
I quite like smartgate, the system used at most international airports in Australia & New Zealand (not just SYD).
Each face-recogniser has its own line. It is generally very quick but can still get lengthy queues at particularly busy times.