If your system is working properly with both switches in the position you show (down) and the light is on, then by assumption you mean with up the switches in the up position your light is off --- as EFL says --- not possible
Sorry, it seems a matter of talking at cross-purposes - for the OP, both downstairs light switches are turned 'On' and both landing and hallway lights are illuminated - exactly what they want.
We are talking about the landing light being illuminated, when the hallway switch and the landing switch are either both on, or both off.
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