I found this image in a research note published in the Journal of Arachnology (1995 23:199-201).
Something weird must have happened developmentally to cause these individuals to have two tails/stingers (metasoma/telson). Vachon believes the anomaly to be a case of incomplete twinning. Apparently this mutation was known in ancient times, because Pliny, citing Aelian (alien?), placed the two-tailed scorpions in their own class(Vachon 1953). Modern literature does not describe these beasts until Pavesi in 1881. Since then two-tailed specimens have been described in at least 9 species of scorpion. I've heard at one point the two-tailed specimens were considered a completely different species, though this could be a misinterpretation of the point made earlier. Certainly a curiosity I'd rather not be stung/stung by.