Hilchos Nedarim 13:11

Let’s say that a man’s wife makes a vow; he hears it and extends the vow to himself as well. In such a case, he can’t annul it because he has already affirmed it. If the man makes a vow and his wife extends it to herself, he can annul it but it remains binding on him.

Hilchos Nedarim 13:12

Pursuant to the previous halacha, if a man hears his wife or daughter declare herself a nazir and he says, “I am, too,” he can’t annul her vow and they are both nazirs. If he declares himself a nazir and she hears and says, “I am, too,” he can annul her vow and his remains in effect. The same is true in all comparable cases.