If I had to hazard a guess, it would be that the maker of the guitar realized he or she could transfer more energy to the sounding board with the addition of the second saddle.
The saddle further from the frets doesn't change anything tonically, since the mode of vibration is defined by the closer saddle. But it's imaginable that if the sounding board is vibrating significantly, there would still be some energy available between the first saddle and the tailpiece.
Even if this didn't increase the volume, the maker may have decided to do it just because it changed the sound of the instrument without degrading its pitch-performance. Any change is a candidate for that unique characteristic that makes your instrument the one everybody wants to buy!
EDIT: Another possibility is simply that the saddle that the maker was using had only ever been used for six strings before... perhaps he or she was worried it would break under the tension of 12 strings. So, divide those forces in half by using two pieces and no more worries!