Oyeboten, it sounds like you have a real interest in both safety and the historical development of reloading, both of which we can all applaud. However, I think the PERSONAL progression you have in mind is a bit cart-before-horse in terms of both these goals.
In historical terms: When smokeless powder came along in cartridges like the .38 (and it's a great cartridge to do all this with), people did not BEGIN by dicking around mixing it with BP--they REPLACED BP with smokeless and thoroughly explored the potential of the smokeless powders available to them. The mixed/duplex loads were a subsequent and minor development, an attempted refinement.
In terms of safety: modern smokeless powders are wonderfully consistent, thoroughly researched, and easy to work with. Of course they require care, but they are used all the time by grillions of people who still manage to count to ten without resorting to taking off their socks. Duplex loads? Again, they're an esoteric, specialized branch; nobody STARTS their smokeless reloading with Duplex loads, for good reasons. They are poorly documented and understood.
So from both historical and safety perspectives, I'd suggest you do some smokeless reloading before you start mixing smokeless and BP or mixing different smokeless powders. If you want to keep it historical, start with Bullseye, which is as old as commercial smokeless powder gets. You may find that, with all the smokeless options that have been developed, Duplex loads have remained a small historical cul-de-sac for good reason; modern smokeless powders can do anything in a .38, from gallery loads to Keiths.
Hope I haven't misread your interest or intentions.
