Reading out two sensors via Com-Port at the same time I have two Arduino Nano Every and a sensor connected to each of them. Here is what's happening:


I upload the sketch to the first Arduino on Com4 and see its output is correct via the Serial Monitor
I open a new instance of the Arduino-software, upload the same sketch to the second Arduino on Com6 and see its output is correct. However, in the meantime the Arduino on Com4 has stopped sending or receiving data
If I close the second IDE-instance and re-upload the sketch to the first Arduino on Com4, it starts working again.


So, essentially I have two Arduinos which I can't read out at the same time. 

The sensors are not connected, so I don't know where the problem is. Shouldn't the Com-Ports be independent?
Shouldn't the Com-Ports be independent?


Not in the Arduino IDE, no. Basically the IDE is [expletive deleted] when it comes to using more than one board at a time.

You're better off using a proper serial terminal program, not the "serial monitor" in the IDE. Something like Teraterm or Putty, where you can have multiple instances on different ports at once.