Why hasn't anyone tried making two propellers facing upwards and the other two facing downwards? Within a quadcopter, I know usually it's all the propellers facing upwards or all of them facing downwards. Why hasn't anyone tried making two propellers facing upwards and the other two facing downwards? Do some forces cancel out and renders that method ineffective?
People have done that.

And it works.

They do it because it's one of the logical configuration for a quadcopter based VTOL plane.

Example: