Yes, it is more common than not to be on 2, or even 3-meds to control bipolar symptoms. My daughter is on 3, with great success - Lamictal, risperdal and seroquel.
If your pdoc wrote a script for lithium then I'd follow his instruction. As for weight gain, it can happen, but with regular exercise and eating the same amount of food you do now, you shouldn't need to worry. My daughter's pdoc always says "a stable brain chemistry is first and foremost".