The problem you describe is common in kittens. Excessive gas can result from various causes, such as consuming too much food. As you may have noticed, when there are two cats in a home, they may eat simply in the name of competition, and overeating can disrupt the digestive process. Flatulence will often stop over time.
You should, however, make sure that your kittens are free of intestinal parasites (worms). Take your kittens to your veterinarian for a proper health exam, which will include checking the stools for parasites.
If the situation does not resolve itself relatively soon, you may want to consider changing their diet. It's possible that an ingredient in the food you are currently feeding them may be causing the flatulence. A simple dietary change may correct this. And be sure to avoid any table foods or milk products, which can cause digestive upsets.
Parenting 
