Mix: merging two or more sass files

Is it possible to override/merge elements from one sass file with another? I'm trying to make some changes to the default bootstrap, but I like to keep the output as compact and clean as possible. I found this topic on Stackoverflow, but I'm not sure how to do that last step (sass-convert) with Mix.

So let's say I have two files: a.scss:

div{
    font-size: 42px;
}

And b.scss:

div{
    color: red;
}

And overall app.scss file:

@import 'a';
@import 'b';

And my webpack.mix.js looks like this:

mix.sass('resources/css/app.scss', 'public/css')

I'm looking for this output:

div {
  font-size: 42px;
  color: red;
}

But so far I'm only getting:

div {
  font-size: 42px;
}

div {
  color: red;
}

How can I do this last step?