Actually with the 315, there are four dual-band antenna connectors. So it's more than two per band, you need all four dual-band antennas for the AP (same number, but I wanted to make sure that no one thinks 2 are for 2.4Ghz and 2 are for 5Ghz.
Colin is ABSOLUTELY correct, you should be using outdoor APs for outdoor environments. There is virtually no Wave2 benefit outdoors (only within 20-30ft max), due to the nature of MU-MIMO, and you will get overall better performance at range with outdoor APs because they have better radios built for outdoor environments.
By the time you buy an indoor AP, the NEMA box, the bulkhead connectors, the LMR cables to the antennas, the antennas themselves, etc, the labor to install and hang, etc, your 275+mounting bracket will be less expensive AND perform better.