Gulp concat two SASS files into one (using Compass)

I'm having serious brain farts with Gulp. I need to concatenate two SASS files into one. That is easy enough, but I'm using Compass as well and that's what seems to be messing me up. Here's the critical/important stuff I have related to this issue:

// Include Gulp
var gulp = require('gulp'),

// Include plugins
  cache   = require('gulp-cache'),
  gutil   = require('gulp-util'),
  plumber = require('gulp-plumber'),
  notify  = require('gulp-notify'),
  rename  = require('gulp-rename'),

  compass   = require('gulp-compass'),
  minifycss = require('gulp-minify-css'),

  jshint = require('gulp-jshint'),
  uglify = require('gulp-uglify'),
  concat = require('gulp-concat'),

  // imagemin   = require('gulp-imagemin'),

  livereload = require('gulp-livereload');



// Error Handling
// ============================================================
var onError = function (err) {
  gutil.beep();
  console.log( err );
};

// Styles Task
// ============================================================
gulp.task('core-css', function() {
  retu gulp.src([
        'sass/screen.scss',
        'sass/style.scss'
      ])
    .pipe(concat('styles.css'))
    .pipe( plumber({ errorHandler: onError }) )
    .pipe(compass({
      config_file: 'config.rb',
      css: 'public/css',
      sass: 'sass',
      image: 'public/images'
    }))
    // .pipe(rename('styles.css'))
    .pipe(gulp.dest('public/css'))
    .pipe(livereload())
    .pipe(notify({ message: 'Core CSS compiled' }));
});

When I try gulp core-css I get an error that reads File not found: /path/to/sass/styles.css. I'm sure I'm missing something small and obvious, but it's been a while since I've had to mess with Gulp. So I just need a hand with concatenating the two SASS files in combination with Compass.

Thanks!