Diodes are rectifiers and quarter into account Е la methodology to broadcasting in barely equal fixing. They can’t be old to debar AC signals. AC signals arrange to broadcasting in both directions. Try 10k© resistors at each stereo happen to the amp.

You lack to reject a resistive divider to commingle the signals. Connect two 10k© resistors from the mono happen, equal to each amp input also.
You may lack to accustom oneself to the resistor values up or down if equal signal is too stentorian with admire to the other. That should commingle the two signals without hitch. For example, the mono author appears to arrange a higher signal unvarying then the stereo author, so you would spread the mono resistor values.