Reading serial port

Hi, how to read the Arduino data it receives from the second Arduino? For example, when I come: temperature1: 22 ° C, temperature2: 10 ° C. So for example these values summed and send it back as temperature3: ...

Thank you in advance.

how to read the Arduino data it receives from the second Arduino?

Step 1: Connect the RX and TX wires to the same serial instance. Connecting to TX1 and RX2 does not make sense.