It looks like maybe these can come apart by using the notch to pry. Or as a means to unscrew, if it's threaded on. But I doubt that, since the wheels spin both directions, either thread direction would tend to get unscrewed.
Or perhaps the notch is meant to be used to align the two wheels? Unfortunately, the notched side is hidden when assembled and splitting the automatic bridge jarred the wheels enough to destroy any alignment they might have had.