I hate to be a dissenter, but duplexing has been around for a long time. It was quite common with the old time Schuetzen riflemen.
Mixing powders with progressive burning rates was experimented with in the Civil War. It worked, but was too labour intensive to be worthwhile.
I've even read of using a finer grade of BP under the main charge to facilitate burning. Who could bother?
I've played with duplexing in my Sharps 50-70's and it makes for a very clean burning load. It's labour intensive and a PITA to do properly, so I rarely bother, but it works.
Now I'll duck the road apples comin' at me ......