Probably better to SEND just one or the other. Then, in the Arduino which has received THE TEMPERATURE, you can present the information in both forms, C and F. It may pay you to use a "bespoke" scale for the temperatures. If you expect, say -10 to 35 degrees C, and you want it to the nearest tenth degree, SEND 0 to 450... and when the number gets to the receiving Arduino, subtract 100 and divide by 10, to turn the number into degees C.... probably quite late in the day. All positive integers are a lot easier to work with than a mixture of +ve and negative, and fractions. Turn "the number" into degrees Celsius, at the last minute, just for the poor weak minded humans reading the display.