Do you get the fruit wet when you water? Splitting skin can be caused by water on the fruit and also irregular watering when you allow the soil to dry out and then drench it with water. Tomatoes like consistent water and sometimes this is harder to do when they grow in pots. Try using a potting soil with water holding abilities (I use one that also includes fertilizer made from Miracle Grow) add some compost to your potting soil to absorb and hold moisture. Then use a long-spouted watering can to moisten the soil and not the fruit. I see beautiful tomatoes in your garden this summer. Keep growing, Marianne Binetti