2+2 dialogue is a format of interaction of officials of countries.
Japan has applied it extensively for interaction with multiple countries.
India participated in 2+2 with Japan in 2012 where our Foreign and Defence Secretaries met Japanese counterparts.
Recently India and US had agreed to interact in the 2+2 format at the Ministers level, though the meeting could not materialise twice.
The advantage of this format is that many strategic matters of vital importance which fall under the domain of both the external affairs as well as defence ministries, are comprehensively covered and any issue of jurisdiction and follow up is taken care of. 2+2, as of now, is limited to external affairs and defence ministries but may be tried out by a combination of other ministries also.