Let me just shut down that idea here and now: Swadians will never shoot or throw multiple projectiles at once.
As far as bows with multiple strings go.... That is pretty impractical and there is no historical evidence that I could find of any such thing existing. The "strings" you see on a compound bow are actually all just one cord wrapped around pullies. You only knock the arrows on the segment farthest back. There is a variety of compound bow where the cords in the middle are separate from the "shooting string" but those are, again, compound bows, not period bows.
As far as bows with multiple strings go.... That is pretty impractical and there is no historical evidence that I could find of any such thing existing. The "strings" you see on a compound bow are actually all just one cord wrapped around pullies. You only knock the arrows on the segment farthest back. There is a variety of compound bow where the cords in the middle are separate from the "shooting string" but those are, again, compound bows, not period bows.