Torque of the rails or torque of the post? I am building a bike and I am  using a saddle with carbon rails on an alloy seatpost. I read that it's not an issue as long as the shape of the clamps is compatible with the shape of the rails, and in my case it is.
However, the torque specified on the rails (6 Nm) and the torque specified on the post (12 Nm ~ 15 Nm) don't have any overlap. Which of the two should I go with?
I am afraid that if I go higher than 6 Nm, it will damage the carbon fiber of the rails, but if I go lower than 12 Nm, the saddle will slip while I ride, and as a consequence will again damage the rails.
Should I ditch the combination all toghether? Or something in the middle, say 9 Nm, is going to work just fine?
If the max. torque on the rails is 6Nm then you shouldn't exceed that in no case. The 12-15Nm is the maximum when the clamp is used with metal rails.
In your case, start by taking the clamp apart and clean the bits that touch the rails of your saddle from any remains of grease with a solvent. Check the clamps for sharp edges and give them a sanding with fine paper if required. Then apply some carbon mounting paste, grease the bolts of the clamp and tighten to the recommended torque for the saddle.