it’s pretty easy really - just define a new serial port using COM2 and you’re done. https://www.ghielectronics.com/docs/15/uart-serial is the docs page but I don’t know how accurate it is for 4.2 (or 4.3) but that’s a pretty core function and I doubt it has changed.
Whether you do this in a Gadgeteer app or not is pretty much irrelevant - just make sure your SerialPort object is outside the ProgramStarted() method and you should be fine.