Right now, It's not possible to define users providers in two (or more) files.

It's caused by

It was introduced by 0b8fef2#diff-11df2572a68def120496f57a8eeff7afR184 (looks like there was no PR)

Actually, I don't really understand the rational behind this constraint.
I have commented this line in my project, and everything works fine.

The PR is quite easy, I can submit it as soon as you're OK with that.