bascially you are playing chords. So when you see 67, you play a blow 6 & blow 7 at the same time. -67 means draw 6 & draw 7 at the same time. At least that would be my guess.
But some of the songs that I have tried to tab seemed to have difficult chords for a harp... so I either omit something, improvise , or just not tab that particular song.
I can tell you from playing the song myself that those are meant to be played at the same time.
When I was learning to play the song I noticed that it was the double notes like that and have wondered if Billy Joel was a novice when he wrote the song (a common mistake when beginners are playing) and just accidentally was playing the double notes or if he wrote them in there on purpose.