My parents divorced and each remarried. My stepmother (my father's wife) adopted me through a simple adoption. Can my stepfather (my mother's husband) also adopt me?

No. According to Article 345-2 of the French Civil Code, a person cannot be adopted by more than one individual, except by two spouses, partners in a civil solidarity pact (PACS), or cohabitees. The French Supreme Court (Cour de cassation) confirmed this in its ruling of 12 January 2011 (Case No. 09-16.527), stating that it is not legally possible for the same person to be subject to two successive simple adoptions by each of their stepparents.