I wanted a second temperature sensor on my Sonoff TH10. I had read the blog post from Xose "Tinkerman" Pérez and decided to set about modifying my switch.
In TASMOTA, configuring to have a second temperature sensor is straightforward and requires no software modifications. I've gone Main / Configuration / Configure Module and selected Si7021 as the GPIO4 temperature sensor. The Configure Module and Main screens now look like this: