First base is occupied. Batter hits the ball and is thrown out at first (first). Does the runner have to be tagged at second or is it a force out.?
After the runner is thrown out at first, the runner who was on first is no longer forced anywhere. He must be tagged.
There are two reasons why it is best to throw out the lead runner first:
If the runner is not forced at the next base, he must be tagged. If you throw out the runner behind the lead runner, he is no longer forced to advance (he could return to the previous base), and you create extra work for yourself by having to tag him.
The lead runner is the closest runner to home plate - i.e. the closest to scoring.