Yes. A line between two boxes is the side of both. If each already has three sides, drawing it claims both and you still move again.
Yes. Every line inside the grid is the side of two neighbouring boxes. If both of those boxes already have three sides drawn, the line that completes them closes both at once and you score two boxes with one move.
What happens next
You still get only the usual bonus move: after closing a box you draw again, whether you closed one or two. Border lines touch a single box, so they can only ever close one (see the board size).
Setting up a shared line for yourself is a common trick, and avoiding giving one to your opponent is a good habit. The strategy guide has more.