My pdoc added Topamax while I was still taking Lamictal. I still take both. Topamax has been my wonder drug. It leveled out my moods better than anything else and I lost close to 70 pounds.
My pdoc understood how the weight gain was a cause of my depression, so she wasn't hesitant at all to put me on Topamax. In fact, she just increased it because I was eating alot and gained some weight ( and I mentioned I had ups and downs).
It has it's side effects, but most go away after a while. It decreases the effectiveness of birth control pills, too. My husband had a vasectomy to solve that problem.
So I would definately give it a shot. You could stay on both drugs at the same time.