The import function assumes just one provider, and will try and common up files (like ca.crt, pass.txt, etc) that are supplied.
You can absolutely use multiple providers if you ensure the paths in the files are all correct (and absolute). For example you might have one ovpn file with "ca .../provider1/ca.crt" and "auth-user-pass ../provider1/pass.txt" and another ovpn file with "ca .../provider2/ca.crt", etc.
If you use the import wizard to import just one provider, you should see the resulting files, which can then be duplicated/modifed. There are some words on the github wiki about this too.