Years ago, my mother remarried, and her husband adopted me (simple adoption). I’d like to give them both some money. I know my mother has a tax allowance. Does my stepfather benefit from one too?

No. For tax purposes, a simple adoption does not create a donation or inheritance tax allowance for the adopter (Art. 786 CGI), with few exceptions, which apply only to the adoptee. Therefore, your stepfather will pay 60% tax on the full amount (Art. 777, IV CGI).