You're right. The AP6212A module does not offer a control pin to control RF switch, which is different from the module on Photon. And it's a pity that we didn't pay much attention to the RF switching characteristic before production.
But it doesn't mean that the Duo can not use an external antenna. We solder a RF connector so that you can use it with some additional changes to the board as stated above. I know it means a lot to some software developers. . I think there is little chance that application need to switch the external/internal antenna frequently.
Since both WiFi and BLE share the same antenna, when the antenna works in its best state, both the TX power and RX sensitivity of WiFi and BLE are in best performance. While you should know that the antenna is responsible for converting the high frequency on-board signal to wireless electromagnetic wave and vice versa, the power amplifier (PA) in the wireless chip determines the strongest strength of RF signal. And the PA of WiFi and BLE is independent in the wireless chip, so even the antenna works in its best state, the TX power and RX sensitivity may be different. Please figure me out if I phrase incorrectly.
Following is the framework of the wireless chip: