Reading out two sensors via Com-Port at the same time
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.