I dont think you can with the ewelink s/w and onboard firmware, you can fit a 2nd sonoff relay into the fan power supply, then add that sonoff device to the same group in your ewelink app Then you will be able to create a scene in which the 2nd sonoff relay can be controlled by either or both the temperature or humidity readings the TH16 device takes.
I know after buying a th16 with the intention of using it to switch the central heating system and spending way too long attempting to do it with the hi amp earthed relay connection of the th16!
if prepared to get stuck in with some modding there is an alternative firmware
(hope its ok to post the above ? apologies if not..)