I saw some graphs from the old shooters.com board from someone that had bought a transducer system and tested blackpowder, smokeless and duplex loads in his rifle.  The duplex loads at low smokeless ratios were interesting, showed a quick pressure rise, then a pressure spike, then a slow pressure decline.  Essentially the blackpowder pressure, then a smokeless explosion, then the normal blackpowder pressure decline.

As I recall, as he increased the duplex charge, the loads showed steadily rising pressure spikes to about 2 to 2.5 times blackpowder pressures until 10% smokeless is reached, and then the graph looks essentially the same as loads with that much smokeless and a filler; maybe just a bit slower pressure decline from the peak than you would get with smokeless alone.  The shooter that had done the testing had the opinion that duplex loads produced HIGHER pressure spikes than smokeless loads alone that produced the same velocities.  His take on it was that the blackpowder ignited faster and built the base pressure of the smokeless so that it effectively burned faster than it's normal burning rate.  His conclusion was that duplex loads were a bad idea for original guns.   

I'm not sure I agree with that conclusion, my take on it was keep the duplex down to way less than 10% of the blackpowder capacity and stop duplexing with double base shotgun powders; use slower rifle powders.  Soon after that I went to straight black anyway, as I found it worked better.  I just had a slow transition from smokeless small bore rifles to blackpowder big bores because of my preconceived notions about blackpowder.

dave