Under Settings > Units > Temperature, the only two choices given are Fahrenheit and Celsius. There is no choice given to display both simultaneously. Here are two screenshots for proof:
Yes, because that's setting the units for the entire app. There's no explicit setting to just display both simultaneously, you have to add the other unit type to one of the customizable slots (which are configured via the customization screen). Most people who want to see both units don't want to see both in every single place where temperature is displayed - that's why I give the option to choose where it appears.
And like I said before, this will be made more obvious in the redesign, so no real point arguing about this here.