Bob and Mike Bryan decided to retire prior to this year's U.S. Open. They have to go down as one of, if not the best, doubles teams of all time.
Watching them play helped me teach my students how to cover the ball hit down the middle in doubles. It always seemed like there were no dividing lines on the tennis court when these two played together. Both players were always covering the middle and, based on the different situations, each took their turn making incredible shots on balls that would have gotten past 99 percent of the players who have ever played the game.
There are many different answers to the "Whose ball is it?" question. The person with the forehand, the person closer to the net, the more aggressive player, the better volleyer, and many others, have been used by pros over the years. My advice is for both players to move and play as if every ball is their responsibility. Communicate with each other after the point if you feel that the shot down the middle was not covered as well as it could have been. Always be ready to hit that next volley and don't divide the court in half using the center strap or center service line as the determining factor.
Taking the ball down the middle is the responsibility of both players and doing it well is one of the keys to winning the hotly contested points at the end of the match.
Thanks Bob and Mike!