If you need a second burner on, I suggest you go with the front and back. Keep the heat indirect. Also, the burner under the water pan will boil away the water fast. I would fill the pan 3/4 full and keep an eye on it.
I expect with a burner on high and one on medium the temp will continue to rise in the grill so keep an eye on it. You will likely have to turn down one or both after the grill fully heats up.