Can you add a second light off an existing switch?

You can add as many light fixtures as you want to the existing switching setup, provided that doing so does not cause you to overload your circuit. To be safe, identify which fixtures and receptacles are on that circuit, and make sure the total connected wattage is no greater than 1440 watts (for a 15A breaker) or 1920 watts (for a 20A breaker). These values incorporate a safety factor by only allowing the circuit to carry 80 percent of the circuit breaker's value. If the total connected wattage is (or will be, by adding the new fixture) over that limit, you'll need to call an electrician to get another circuit and split up your load.