It does keep the bore clean (no blow tubing needed), and it gives you more MV. I like duplex loading for 3 applications: Hunting, offhand target shooting, and very long range shooting with the smaller cartridges.
The hunting application is obvious. In offhand target shooting, it can be difficult to manipulate the rifle and the blow tube, while also trying to rest and relax for the next shot. And if you are in a relay with a lot of smokeless shooters - you could also be pressed for time! (I'm not talking about NRA silhouette here - where duplex is not allowed).
Long range shooting (over 800 yds) is probably best done with a 45/90 or larger. If you want to be competitive with a 45/70, 40/70, 40/65, etc - you will probably need to duplex to get the MV up to 1300 fps or maybe a bit faster - and it is generally accepted that this MV is needed for success at the longer ranges.
The NRA allows up to 20% smokeless in their LR matches. I think you should start at about 5% smokeless and see if that gives you what you need. The way to do it is to take your full black powder load (say it's 60 grains) and then take 5% of that in smokeless and reduce the black that same amount. So that would be 3 gr. smokeless and 57 gr black. You can work up to about 10% (6 gr smokeless and 54 gr black). I have never gone beyond that. Put the smokeless in first. The long-time standard for the smokeless is SR 4759, and it is excellent, but Reloader 7 is supposed to be about as good. I would stick with one of these two.
Duplex is safe but it does give higher pressures - only use it in a strong, modern firearm - not a Trapdoor Springfield, or anything like that.
