I'm not a worm guy, so i'm just guessing here. But why just a couple, why not ten (or so) in each growbag?
My other thought is, would they survive the watering process? Dumping a gallon of water in the bag, relatively quickly, is a lot more vigorous than rainfall - the soil is, literally, flooded. Would they dig thier way out in time, or would they get drowned out? Or would they just be able to survive those watery conditions until the water passes and oxygen is sucked down into the soil?