In Postgres, apparently there is no way to specify multiple columns in one expression.

Something that Postgres does support is column numbers:

SELECT "time", "dwell_time", "triggering_event"
FROM beacons
WHERE transmitter_name='beacon_x2_y3'
AND "time" LIKE '2015-08-18%'
ORDER BY
	1 ASC,
	3 ASC

This will return results ordered by both time, and triggering_event.