Combining power-ups helps you clear more cells and produce a bigger bang. Here are the combinations currently in the game:

Chips+Chips doubles the radius of the Bag of Chips effect.
Chips+Soda removes three rows and three columns of pieces.
Soda+Soda removes one row and one column of cells, regardless of the direction the Cans are pointed in.
Chips or Soda+Spinning Top works like a regular Spinning Top and makes the Spinning Top take the second power-up along with it to a random cell.
Spinning Top+Spinning Top produces three Spinning Tops that target different cells.
Bucket of Candy+Soda or Chips or Spinning Top makes the Bucket of Candy turn all the pieces of the same color as the matched power-up into a power-up. For example, if matched with a Blue Can of Soda, the Bucket of Candy will turn all the blue pieces into Cans of Soda.
Bucket of Candy+Bucket of Candy removes all the pieces from the field, clearing one obstacle layer in the process.