Our stoker has a large TOPEAK Aero Wedge Pack w/ straps hanging under the telescoping bar. There isn't room to clip a bag beneath the captain's saddle, otherwise we would have went that route. The stoker's saddle always has a bag clipped under it, but accessing its contents on the go isn't nearly as safe.
One note, since that wedge pack is expandable, I placed a small strip of electrical tape on the bar beneath the bag where the zipper rubs against it ever so slightly.
D