• Thank you very much for you advice, Gordon!

    Doing LoopbackA.setConsole() on onInit works great! Serial1 is quiet now ;-)

    you could make a handler for USB.onData that would swap the console
    back to USB when you pressed a button.

    I couldn't fully understand your suggestion.
    Do you mean

    USB.onData(function (e) { USB.setConsole(); });

    It seems overkill to on every character received on USB.
    Or perhaps you meant assigning USB.setConsole for a press of button?

    function onInit(){
      setWatch(function(){ USB.setConsole();}, BTN, true);

    Anyway, above onInit function seem to behave as I wanted.
    (just have to remember to call onInit or LoopbackA.setConsole(); on WebIDE before unplugging USB.)