Why you can't use two thermometers at the same time
You can only use 1 basal thermometer with Natural Cycles, as two thermometers at the same time may cause too high variations and therefore affect the algorithm negatively.
To be more elaborate, our thermometers are ±0.05 °C accurate. If one of your thermometers is showing +0.05 °C and the other -0.05 °C, there will be differences of 0.1 °C. The app is looking for the smallest differences when trying to detect ovulation, and having two thermometers can cause false temperature fluctuations, even if both thermometers are from us. This incorrect data can cause too high fluctuations, which will make it difficult for the algorithm to confirm ovulation and you'll be given more red days.
So, you can not have a thermometer at home and another one at your partner's house or similar. You must use the same thermometer at all times! If needed, you are only to change thermometer at the beginning of a new cycle.