Though the datasheed speaks about 12Bit / 14Bit, it doesn't say much more about the internals of the IC.
It is possible that there are two ADCs, because the principle of measurement is completely different. (Temperature: Measure resistance / voltage; Humidity: Measure capacitance)
It is also possible that there is just one ADC. Even if 14Bit doesn't make sense for humidity one could ask why they put in the effort to cut away two bits. Maybe, there is a single 14Bit ADC, and the voltage range of the humidity sensor is just 1/4 of the max ADC range.
Finally, each electronic part is a black box, and all you (have to) know is written to the datasheet. The exact internals may be completely different.
By the way, this IC is strange. It uses
SDA as pin names, the first being the selector for temperature / humidity, the second for PWM output...
Though they write about microcontroller, ADC and one-time programmable memory for calibration constants, this device could easily be realized with analogue design, without any ADCs etc.
The resolution of the ADC equals 0.01°C. But the repeatability is just 0.1°C, and the accuracy is 0.3°C. So, even 11Bit would be good enough for temperature measurements.