People used to do it. It was quite common to introduce a bit of faster powder into the case containing a slow, hard-to-ignite powder in order to get consistent ignition.
Nowadays, we have better primers and better powders.
From what I understand there are several problems in getting consistent pressure/time curves. (Essential to avoid pressure spikes that tend to destroy firearms and injure or kill those nearby.)
Mixing of the powders. Sometimes you want the fast powder near the primer and the slower powder further up. But it is hard to keep the two from migrating into one another.
Unmixing of the powders. Sometimes you may want to have the two powders mixed uniformly. But the nature of powders is that larger granules will tend to separate from smaller granules.
If you mix the two powders before loading, you don't know how much of each powder is going into your weighed powder charge. Very difficult to control the proportions.
If you mix by adding a specific amount of each powder into individual cases, you know the proportions, but still don't control the mixing inside the case.
You can prevent mixing inside the case if you separate the powders with a wad, but then you introduce the additional effect of an extra component and its unknown effect on the pressure-time curve.
You can limit the mixing effects by using a 100% or compressed load.
So, it can be done safely. Professionals used to do it all the time. But they ALWAYS had laboratories, pressure test barrels, protective barriers, lots of time and money.
THIS SHOULD NEVER BE DONE WITHOUT A BALLISTICS LAB!
What you are contemplating is just short of brewing your own propellant on your kitchen stove.
In a word. Insane.
A reasonable question, yes. But insane to contemplate doing.
Respectfully
Lost Sheep
