Duplexing has been going on longer than any of us have been alive. There is even data listed in a Lyman manual for BPCR using no more than 10% smokeless in older cartridges like 45/70, 45/90 ect.
The early duplexes used 4F under 1.5F or 2F and later powders like 4759 and 5744 started being used as the "igniter" charge.
Google "BPCR duplexing". You will get many hours of reading on the topic.
BTW Hodgdon has been dong the same thing since Pyro pellets came out. What do you think the black end of a Pyro pellet is made out of? Right in the Pyrodex MSDS it lists Pyro pellets as having a lower ignition requirement.
https://www.hodgdon.com/PDF/MSDS%20F...Sheet-2013.pdf
Granular 740F
Pellets 500F