The antenna connector functionality differs depending on the Intel Wireless adapter you are using. For below dual-band Intel Wireless adapters, both antenna connectors are used for Wi-Fi supporting 2.4 GHz and 5 GHz bands. The difference is that one of the antenna is shared by both the Wi-Fi and Bluetooth radio.
Intel Wireless Adapter
Intel Wireless-AC 9260 Intel Dual Band Wireless-AC 8265 Intel Dual Band Wireless-AC 8265 Desktop Kit
Antenna Connector Functionality
Antenna 1 (Aux) Wi-Fi + Bluetooth Antenna 2 (main) Wi-Fi
Note that these are 2x2 Wi-Fi adapters. This means both antennas need to be connected to achieve the optimal Wi-Fi performance.