How do you write Dear Mr and Mrs xxxx in French?

plural (persons of different gender): Chers Monsieur et Madame XXX,

one man: Cher Monsieur XXX / Cher M. XXX, (abbreviated form is M. and not Mr.)

two or more men: Chers Messieurs XXX et YYY, / Chers MM. XXX et YYY

one woman: Chère Madame XXX / Chère Mme XXX,

two or more women: Chères Mesdames XXX et YYY, / Chères Mmes XXX et YYY,