I agree with @PapaMike, although the choice ultimately hinges on the desired feel for your pillow. Are you aiming to replicate a specific pillow’s sensation, perhaps one with a slight bounce or a firm yet soft support?
Personally, I’m not keen on blending both fibers and then stuffing the pillow, despite it being a method some opt for. Adjusting it later if it’s uncomfortable can be quite messy if you dont like the feel and want to separate the fiber mix.
As I previously mentioned, I opted for lightweight 100% cotton zippered “liners/cases” and filled each with the material of my preference. There are various approaches you can take.
One option is to create three chambers: two outer chambers with the same material and an inner one comprising the core.
Another option is to create a two chamber pillow with a different feel on each side.
Some pillows utilize wool batting for the outer layer and latex for the inner core.
Alternatively, you could use wool bolas for the outer chambers and latex for the center, or reverse the materials.
You can always mix the two fibers together for more uniform comfort feel.
In the case of my horsehair pillow, I have two chambers. Flipping the pillow allows me to switch between the latex/memory foam side for a softer feel with the firm support of the horsehair beneath, or for a firmer sensation with a slight springiness.
I am planning to create on with horsehair as the core and the outer portion with a wool batting or bolas, just have not decided yet.
There are numerous combinations to explore, each resulting in varying feels that will require varying weights. If you intend to simply blend the two fibers, the weights suggested by @PapaMike should suffice. I’d recommend making some extra stuffing, just in case adjustments are needed for your comfort preferences.