You would have to calculate the other value yourself. You could do that in a rule and set a Hub variable for the other value, and then use that Hub variable to display the other value on the dashboard.
You could also create a virtual temp sensor device, and have your rule set the calculated value for the virtual temp sensor, and display that temp sensor value on the dashboard.
Edit: As far as one tile goes, you can combine values into one tile with apps, like this one:
You could also work with CSS to overlay the tiles into one tile.