sharing the thermistor between gas gauge and charger

Sharing the thermistor in the battery by both charger and gauge is not impossible, but is very tricky, and not recommended.  You will have to synchronize and isolate the measurement, two measurements at the same time will produce an incorrect temp reading. The biasing circuits are different, so the same thermistor cannot be connected to both ICs without using isolation, such as with an analog switch. 

Connect the battery thermistor to the gauge to ensure the accurate temperature measurements of the cell for accurate gauging.  The gauge also uses the thermistor for detection of battery insertion and removal. If you don't want to add a thermistor for the charger, then connect the fuel gauge BAT_GD pin to the charger CE pin. 

-Leonard

In the case of the BQ27200, it does not need an external thermistor.

The bqJUNIOR uses an integrated temperature sensor to monitor the pack temperature and is reported through
the internal registers. The temperature measurement is used to adjust compensated available capacity and
self-discharge capacity loss.

The BQ24050 does need an external thermistor.