Well if the foul occurs during a throw-in, then the point of interruption is the throw-in...meaning the next throw-in spot won't change. The throw-in doesn't end until it is legally touched. So while you were right that it would go back to the throw-in spot (which I'm guessing was your position), you were right for the wrong reason. It has nothing to do with ball location in this situation but rather what the POI is.
2013-2014 NFHS Case Book
4.19.8 SITUATION F: A1 releases the ball on a throw-in, and before it is legally touched, A2 and B2 commit fouls against each other.
RULING: When a double foul occurs, play is resumed at the point of interruption. Since Team A's throw-in had not ended, the point of interruption would be a throw-in by Team A. (4-36-2b; 10 Penalty 1c)