We had someone robin hood an arrow about a month ago at 70 metres, it happens quite a lot. We were running a beginners course with some kids on it at the time who had all seen Brave (The girl Robin Hoods an arrow in one scene), they thought it was excellent when I showed them the arrow. Generally when it does happen it goes up on the wall in our club house so we have quite a few of them, some still with the second arrow embedded.
With a wooden arrow you just tend to shoot the nock off and the second arrow deflects from the taper, if you use self nocks I imagine it could be just possible to split a weak shaft along the grain of the arrow if you have incorrectly nocked them parallel to the grain and you hit the first arrow perfectly.
Great Deals with : Dave Dee, Dozy, Beaky, Mick & Tich