For a Vishista-advaitist, Sri Krishna and Durga constitute the two Supreme Divine aspects of the coin of God (Bhagaban), the Creator of Brahma-Nature with its rajasic, sattvic and tamasic guna consciousness attributes and interactions for union with.
The two aspects of God govern truth and knowledge for Sri Krishna and dharma and dharmayudha to counter evil for Durga respectively. The Almighty Bhagaban lies dormant in Brahma-Nature but in times of need emerge as the Divine Spirit for the purpose of restoring order in Brahma-Nature when it is in peril through the preponderence of adharma (lack of righteousness)