Simply tap the gear icon top right in your data screen then enable 'Temp two' in your items to track (please note that you may have renamed this temp).
Now you can record two sets of temperatures in your data screen and on your chart screen you'll have the option to toggle between viewing 'Temp' / 'Temp two' / 'Both temps'.
You can also rename your temps if you wish by tapping the gear icon next to 'Temp' or 'Temp two' within your data screen.
Happy charting!