http://www.youtube.com/watch?v=yGHge4FQ8aM&NR=1

Starting at 1:50 the voiceover says "We will suffer terrible losses but you must drive home the attack", during this a pillion rider on a  horse uses a sling to bring down a Roman cavalryman.

The rest of the clip is reckoned to be historically accurate, so is there any primary source for two-up cavalry slingers (or archers)?

I have a feeling that the speech Hadrian gave to assemble troops at Lambaesis, and inscribed on stone (and found and published) mentions mounted slingers