Divide players into two teams ... X and Y. Team X competes against Team Y.
The ball is thrown criss-cross down the line to every other player. Then the ball is returned back to the same partner.
The winning team is the one which gets the ball back to the start first.
Alternate the drill by throwing short hops or increasing the distance between players and/or the throwing distance.
Another alternative is to use one ball and throw the ball to anyone in the opposite line except the player that threw the ball to you.
Reprinted with permission from A Softball Coaches Tool Kit