Neapolitan

You are not suppose to add them up to get six just like you don't add the notes in the treble and bass clef to get six quarter notes in a single measure. It's simply B D B. The last bar in the bass clef can be written differently with rests and ties between quarter notes. But to make it easier it's written in two parts: on top a quarter rest then half note, on the bottom a half note then quarter note. The two parts are played at the same time. The B D B are played on the beat but you hold the B while you play the D then the play B again on the third beat.

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