How do I detect two simultaneous button presses?

Hi! It's basically impossible for you to press both buttons at exactly the same time, so instead you just need to shift thinking a bit - call the same function when either button is pressed, then check the state of both in that function:

function check() {
  var a = BTN1.read();
  var b = BTN2.read();
  if (a&&b) console.log("Both pressed!");
}
setWatch(check, BTN1, {repeat:true, edge:"rising", debounce:20});
setWatch(check, BTN2, {repeat:true, edge:"rising", debounce:20});