Do I really need to explain why the side bend with two dumbbells is a worthless exercise? Probably, because I still see it every week at the gym!
Listen folks, it's simple mechanics: holding a dumbbell in each hand negates the need for the muscles to do the job. Instead of having to use your obliques to lift the weight from one side up, you simply rely on the weight in the other hand to counter-lever your way up.
It's like a teeter totter. If you have a tiny kid on one side and a fat kid on the other, the tiny kid will have a hard time making the fat kid go up. He'll have to use muscular effort (slamming himself down the seat) to do the job. But if both kids are the same weight it requires almost no effort to get started, and once started pretty much no effort is needed.
Side bends, even when properly done (one side at a time) are an overrated exercise anyway and could be potentially risky for the spine. Try the side-plank crunch instead.