For informations sake since you were wondering - the idea is put a bit of a fast burning powder at the bottom of a casing, under a usually compressed load of very slow burning powder. The idea is that the fast burning powder is easily ignited by the primer, and helps the primer ignite the main powder charge.
Now the basis for this was not modern powders. These were old powders that were very temperamental, at times hard to ignite, and usually compacted - and quite literally needed help to have good ignition.
The "new" usage for duplex was with people who think that they can use a whole lot of slow burning powder in a case, which gives them high velocities with the lower pressures of slower burning powders, and a fast powder to make faster and hotter ignition of that slow powder.
The problem is in that last sentence. Faster and hotter ignition = higher pressure. Every time. Magnum primers = higher pressure, faster powders = higher pressure. So then you are taking all of that pressure, and putting it behind an already large powder load, and you are destined for failure. Catastrophic failure.
Since there is no way to know how the two different pressure curves will align in your case during ignition, there is no way to know when the pressure will spike and how high it will spike. With the literal hundreds of modern powders as their are out there, you have so many options and so much to play with within safe and tested reign, there is no need to test duplex loads.
Now if you are trying to recreate an old pb load known to use duplex loading, because that is a hobby of yours, that might be a reason. But if you are just making a load for shooting and hunting out of a modern firearm with modern components, there is no reason, and there is no gain.
