It's a dual-band antenna; i.e. each antenna element inside the antenna unit serves both the 2.4 GHz and 5 GHz radio simultaneously. There are two elements in the antenna, one per pigtail. This is why there are two antennas required, to support an AP with 4 antenna posts. For APs with dual-band antenna posts (i.e. MR84) it's important that both antennas cover the same area - as client devices supporting multiple streams will attempt to use them, to build a higher bandwidth connection. If used with APs which have dedicated antenna posts for each of the 2.4 and 5 GHz radios (MR74), you could, theoretically, have different antennas attached, covering different areas.
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