Combo Work involves a number of different actions at the same time. There are a myriad of combinations to involve all infield positions, each avoiding the other in a safe and effective manner. Combinations are limitless, but depend on how many coaches are available to hit ground balls. Take some time to script three or four segments during the team defense portion of practice.
Here is an example with two coaches available: Coach #1 hits ground balls to the third basemen. Third basemen toss the ball back to Coach #1 or the baseballs into foul territory. Coach #2 hits double play ground balls to the shortstops. The second basemen turn the double play and the first basemen receive the throws (6-4-3). Here are other variations:
Here are more advanced examples with more coaches and multiple infielders at each position. Each can be executed for several minutes:
For more information, check out my book Coaching the Beginning Pitcher.