The APs could use different antennas, but you'd not get the full benefit with only one changed, it would be worth a test though.
From the install guide, you *could* mix antennas on each AP, one omnidirectional pair and one directional antenna. Though it isn't recommended for optimal performance, it would definitely be worth testing:
2x N-Type antenna ports are located at the top of the MR76 and 2x are located at the bottom of the MR76. The 4 antenna ports are connected to 2.4 GHz and 5 GHz client serving radios respectively. Please see the markings on the AP to verify 2.4GHz and 5GHz ports. In order to ensure the highest performance, ensure antennas connected to the 4 ports have overlapping coverage areas.
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