If the cause is watering, then wouldn't the best solution be to be extremely consistent as far as watering goes...choose a schedule for watering for the plant/plants involved, and water those plants with a small amount of water to begin with...and slowly increase the amount of water offered to the plant until you get it to a healthy amount. You'd have to deal with continued splitting until the plant acclimatizes, but you're better off in the long run. I personally would be afraid to cut the roots of a plant if there's a chance of another dry spell. Yeah, having a schedule like that will require taking down records of your water times and amounts, but it would also get you into the habit of keeping a close eye on your plants, and you'll be more likely to notice when something odd changes about them--and of course, consistent watering (healthy amounts and incriments of course) can help maximize your crop yield. Consider also how much water is in a tomato--they need a lot of water. Sounds like if the plant's sucking up so much water to put into the tomatoes that it splits them, that it's not getting enough.
I'm no botanist of course...but that's what common sense tells me.