Correlated signal from two sensors in one circuit
If I've found the same humidity sensor, it has pretty big output impedance and the maximum input impedance for the ADC input is 10kOhms. According the the datasheet the recommended minimum load for the sensor is about 80kOhms (?). That means readings of humidity sensor will be highly affected by sampling capacitor initial charge (= the value of previous reading).

Basically reading GND between sensor readings helps to separate the results from each other, but the humidity sensor won't be able to charge sampling capacitor to its output voltage without amplifying the current (you'll get values much lower than it should be).

Or you can repeat the readings on humidity sensor channel several times until the value is more "stable".