If I understand your question, what you want to do is add another sort field.
var sortField1:SortField = new SortField( "year" );
sortField1.numeric = true;
var sortField2:SortField = new SortField( "model" );
sortField2.caseInsensitive = true;
sortField2.descending = true;
collection.sort:Sort = new Sort();
collection.sort.fields = [ sortField1, sortField2 ];
The above code should do the trick, sorting first on model then on year.
The following documentation may be helpful:
Let me know if this helps.
That was it...
It works if I have two items in my sample data.
It doesn't work if I have 6 fields in my arrayCollection and I'm sorting base on 5 and 2 field.