After careful consideration, I decided to develop a special mixin that enables sorting in multiple directions. This extension of the SortableMixin is designed to be as backwards-compatible as possible. Essentially, it functions like a typical SortableMixin, but with the addition of the sortAscendingProperties property. This property is an array of sort property names (members of the sortProperty array) that should be sorted in ascending order. If a property is included in sortAscendingProperties, it will be sorted in ascending order; otherwise, it will be sorted according to the sortAscending value, which serves as a default sorting direction.
I chose to name this mixin MultiSortableMixin, although I acknowledge that it may not be the most fitting name.

(function() {

    var get = Ember.get, set = Ember.set, forEach = Ember.EnumerableUtils.forEach;

    /**
     * Extends the SortableMixin by allowing sorting in multiple orders.
     *
     * sortProperties - array of strings
     * sortAscending - default sort order
     * sortAscendingProperties - properties listed here will be sorted in ascending order, while others will follow the default sort order
     */
    Ember.MultiSortableMixin = Ember.Mixin.create(Ember.SortableMixin, {
        /**
         Specifies the arrangedContent's sort direction

         @property {Array} sortAscendingProperties
         */
        sortAscendingProperties: null,
        orderBy: function(item1, item2) {
            var result = 0,
                    sortProperties = get(this, 'sortProperties'),
                    sortAscending = get(this, 'sortAscending'),
                    sortAscendingProperties = get(this, 'sortAscendingProperties');

            Ember.assert("you need to define `sortProperties`", !!sortProperties);

            forEach(sortProperties, function(propertyName) {
                if (result === 0) {
                    result = Ember.compare(get(item1, propertyName), get(item2, propertyName));
                    //use default sortAscending if propertyName is not listed in sortAscendingProperties
                    var sa = (sortAscendingProperties && sortAscendingProperties.indexOf(propertyName) > -1) || sortAscending;
                    if ((result !== 0) && !sa) {
                        result = (-1) * result;
                    }
                }
            });

            return result;
        },
        //Overridden to include additional watched properties. TODO - Since the code is identical to the parent method, explore ways to simply add watched properties
        arrangedContent: Ember.computed('content', 'sortProperties.@each', 'sortAscendingProperties.@each', 'sortAscending', function(key, value) {
            var content = get(this, 'content'),
                    isSorted = get(this, 'isSorted'),
                    sortProperties = get(this, 'sortProperties'),
                    self = this;

            if (content && isSorted) {
                content = content.slice();
                content.sort(function(item1, item2) {
                    return self.orderBy(item1, item2);
                });
                forEach(content, function(item) {
                    forEach(sortProperties, function(sortProperty) {
                        Ember.addObserver(item, sortProperty, this, 'contentItemSortPropertyDidChange');
                    }, this);
                }, this);
                return Ember.A(content);
            }

            return content;
        }),
    //Not needed in this mixin, overridden to disable functionality from SortableMixin. TODO - Find a way to remove it without impacting other functionalities
        sortAscendingDidChange: Ember.observer(function() {
            //empty
        }, 'sortAscending')
    });

})();

Example of how to use:

App.ThingsController = Ember.ArrayController.extend(Ember.MultiSortableMixin, {
    sortProperties: ['prop1', 'prop2', 'prop3'], 
    sortAscending: false,
    sortAscendingProperties: ['prop2', 'prop3'], 
    //your stuff
});

In this demonstration, the content of ThingsController will be sorted first by prop1 in descending order, followed by prop2 and prop3 in ascending order.

Ember does not come with out-of-the-box sorting functionality, but by examining the code of SortableMixin found here, we can see that it utilizes Ember.compare to compare two entities:

orderBy: function(item1, item2) {
    var result = 0,
        sortProperties = get(this, 'sortProperties'),
        sortAscending = get(this, 'sortAscending');

    Ember.assert("you need to define `sortProperties`", !!sortProperties);

    forEach(sortProperties, function(propertyName) {
      if (result === 0) {
        result = Ember.compare(get(item1, propertyName), get(item2, propertyName));
        if ((result !== 0) && !sortAscending) {
          result = (-1) * result;
        }
      }
    });

    return result;
  },

The Ember.compare function includes a check on the Comparable Mixin:

var Comparable = Ember.Comparable;
  if (Comparable) {
    if (type1==='instance' && Comparable.detect(v.constructor)) {
      return v.constructor.compare(v, w);
    }

    if (type2 === 'instance' && Comparable.detect(w.constructor)) {
      return 1-w.constructor.compare(w, v);
    }
  }

My suggested solution involves:

1 - Adding an extra field to your models that contains a wrapper object of all your sorting properties, for example, "combinedAandB"

App.YourModel = Ember.Object.extend({
a : null,
b : null,
combinedAandB : function(){
  // The following Object should implement SortableMixin
  var comparator = App.AandBComparator.create(this.get("a"), this.get("b"));
  return comparator;
}.property("a","b")

2 - Your ComparatorModel (App.AandBComparator) should implement the Comparable Mixin. Within this comparison method, follow your desired sorting behavior (prop a ascending and prop b descending).

3 - Now you can create an ArrayController and sort it based on your combined property:

var yourModelController = //wherever you may obtain that one from
yourModelController.set("sortProperties", "combinedAandB");

Note: This is just a spontaneous idea I had while reading your requirement. I have not yet implemented this, so it's likely not perfect :-)