Outputs/inputs to Multiple Arduinos in Arduino IDE at once
Arduino IDE is editor and builder. Your sketch runs on microcontroller and the microcotroller doesn't need the IDE to run it. The Serial prints from microcontroller go to com port from which any Terminal app can show them, not only the Serial Monitor build-in in IDE.

Connect both arduinos of same type to computer. They will have different ports. Upload your sketch to the first one, then change the port setting in IDE and upload to second arduino.

Serial Monitor can show you the output of arduino connected to the selected port. Switch port in Tools menu and reopen Serial Monitor to see the output of the second arduino.

To display booth at once, there are some options with different difficulty level:


use two instances of a terminal program like PuTTY Terminal Emulator to show the output
connect the TX pins of arduinos together
send data from one arduino to the second and let the second process them