A quick note, generally speaking we don't recommend any high gain antennas for client coverage, the 2x2-2314/2x2.2714 are mostly for limited mesh purposes in countrys where 5Ghz is disallowed.
To the question on the number of antennas, the 274 is a 3x3 AP, which means there are three connectors per radio. You CAN use a 2x2 antenna on a 3x3 AP, you just put the antenna on ANT0/ANT1, and have to put a 50 ohm terminator on the unused connector ANT2 for whatever radio the 2x2 antenna is placed on.