After some switching about it turns out:
1. To light the green lamp on the left side, both switches must be in the "down" position
2. To light the red lamp, one switch has to be "up" (no matter which one), the other must be "down". You may also see the two switches in different positions or say: they must not be equal
3. You cannot light both lamps at once.

And by using our new technical knowledge we can express it this way:


move on