Glonass,
PAXboy, your statement is correct but I believe that would apply to some type of aircraft or operation, at least, on most jet aircrafts, the doors are usually closed previous to engine start.
That is true but I was thinking of the 'generality' of starts. Admitting of course that Pax cannot always be sure which is starting first. Other than by timing and knowing the typical sequence when a route or carrier is used often.
In the winter it is easier as there is often a more distinct cloud of exhaust from a cold engine than from a warm one in summer. Also, if occupying a window seat towards the rear, the sound of start will be distinct. It is then possible to judge by the other activity or push back, whether it is 2 ~ 1 or 1 ~ 2.
I was also thinking about regional airports and aircraft, where a jet bridge/air bridge is not available/used. In this case, ground staff are likely to be clearing the stairs, chocks and so forth. Also, if air stairs are in use, they take a short while to stow. If they can get the No 2 engine started, this might gain a full minute (or more?) on the sector time.