Level 7
I had similar problems in the past. To process two sass files separately, you need to use an array:
mix.sass(['../frontend/sass/app.scss','../backend/sass/app.scss']);
Now, I have not tried to export to different folders, so I'm not 100% sure how to do that, but you may want to try using an array on that as well:
mix.sass(
['../frontend/sass/app.scss','../backend/sass/app.scss'],
['public/frontend/css/','public/backend/css/'],
{includePaths: [elixir.config.bowerDir]}
);
If that doesn't work, you can always move the processed CSS file with the copy() command:
mix.sass(['../frontend/sass/frontend.scss','../backend/sass/backend.scss']);
mix.copy('public/css/fronted.css', 'public/frontend/css/app.css');
mix.copy('public/css/backend.css', 'public/backend/css/app.css');