What is the proper way to tighten dual bolt saddle rail clamp to stop rail slippage? I have a 2018 Specialized Allez, with Titanium 'Hollow Ti' saddle rails. To secure them to the seat post, there are two bolts opposite each other that both clamp down the same upper plate onto the lower plate that hold the rails in place. There is also a marking for 10 newton meters at the top of the seat post. My question is, is there a proper way to secure these bolts? (What I'm bearing in mind is that the upper plate is being affected by two different bolt tensions simultaneously).

I have had the saddle slip back during a ride twice now, only noticing when my position on the bike feels off, resulting in slightly scratched up saddle rails. I have also treated the 10nm rating as a maximum, and therefore usually tighten to 8nm for each bolt. Sometimes one feels a bit easy to turn while the other can feel like it is being forced, which is why I ask if there is a proper way to do this.

There is nothing applied like to the saddle rails (like grease) that might make it slip.
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