On the jolla page it says extension interface for power in/out. You may not get other half battery as a separate additional battery but you can get a new battery with higher capacity and change it with another one. The power input/output can be used for OHs like solar charger.

Normally I2C only has 2 data lines. Some products has also gnd and vcc pins in addition to data. For example wii nunchuck and motion plus has 4 pins (gnd, sda, scl, vcc). We will see if jolla gathered them in one socket or multiple.