What is the proper way to tighten dual bolt saddle rail clamp to stop rail slippage?
A good rule of thumb when tightening down any multiple fastener connection would be to finger tighten (if possible) each bolt until snug, them apply a tool and alternately tighten by stages slowly reaching the torque spec per bolt. This should provide a more even pressure distribution per side. NEVER fully torque one side while the other is loose, always alternate sides with even torque until THE SPECIFIED TORQUE IS REACHED. DO NOT UNDERTIGHEN, OR OVERTIGHEN. There is a little tolerance built-in to the spec (although possibly not shown (e.g. 10 Nm +/- 1 Nm). I do not have information on the hollow Ti rails, but I would think that they are engineered to handle the clamp pressure. You might reference the saddle manual to see if there is a recommended max torque the rails can handle. Due to shifting weight of the rider and slight flexing of the saddle during riding, I recommend that the torque be rechecked after the first couple of rides to ensure everything is still within specs and check that the saddle position has not shifted (I mark saddle rail orientation once comfortable, front-back and angle up/down to allow for easy verification/resetting). 

Good Luck,
HPL