Drop spreaders are designed to use pelleted materials and do not do well with materials that can be more like dust such as feather meal. A rotary spreader may do better although with even a slight breeze you may have more of that meal on you than on your yard.

kimmq is kimmsr

What do you think about mixing Milorganite with the Feather Meal?

Moisten the Milorganite with a hand sprayer so the feather meal will stick to it. Unless you have been using feather meal every few months over time, it will take a long time to decompose. If organics are slow release, feather meal is glacially slow release. But once you use it a few times, the microbes that decompose it will repopulate the soil and it will work faster in the future.