The all between the legs 2 basketball dribbling drill is an advanced drill that is going to work on ball handling as well as developing soft hands and coordination. By being able to dribble two basketballs at the same time you are developing both hands at the sametime and also making it easier to handle one ball when you get into a game situation.
When you do basketball drills like this you want to really push yourself and get out of your comfort zone. Don't be afraid to mess up or lose the basketball, this is part of the drill. You should be going so fast and hard that a mistake happens every know and then. That way when you get into a game situation and are dribbling the basketball at a normal speed you will feel all the more comfortable.