multiple authors per category

Hi!
I try to get the component up and running... we have 3 authors and we planned a category-tree like
1. Nutrition
1.1. Recipes
1.2. Diets
1.4. Food Intoelrances
2. Food & Beverages
2.1.Recipes
2.2. whatever
...

All authors should be able to post into those categories and also into multiple categories... but it seems like only 1 author per category os allowed?

It should be able to look like this:
"Max" postet into "Nutrition, Recipes, Diets, Food&beverages, Recipes" a post "XY whatever".

Thanks for the advice how we could proceed, thanks!
best
Michael