If I am playing Man to Man and select someone to double team, dos this mean that there are two people guarding this person all the time, or does this mean that when he gets the ball, a second defender comes over to double team that person? I am thinking about always concentrating on weak ball handlers, even if they don't get the ball that much.

They only double team him when he touched the ball.  I use the double team quite frequently.  The unknown is that you don't know what player is sliding over.  It would be nice if you could assign that.