Is your feature request related to a problem? Please describe.

Currently bin/allure requires that both simple-result.json and categories.json are in the same folder.
If I put the files into different folders and pass both of them to bin/allure serve, then the report does not pick up the categories.

The problem of having different data in the same folder looks like "overlapping outputs" to the build systems.
Overlapping outputs incur extra burden on builds systems (e.g. https://docs.gradle.org/current/userguide/build_cache_concepts.html#concepts_overlapping_outputs)

Describe the solution you'd like

I would like to provide an explicit pair of "results" and "categories", so allure could build the report accordingly.

For instance: ./allure serve --results results1 --categories path/to/categories.json --results results2 --categories another/categories.json
An alternative option is to prepare the description in a standalone JSON file and pass it to ./allure serve --input inputs.json

Describe alternatives you've considered

An alternative would be producing files into different folders and create yet another folder that combines all the files right before calling allure. However, it would require extra file copy operation.