Is it permissible to combine the intention of making up for the fasting on behalf of the deceased with the reward of the fasting being for both me and her?

Your fasting on behalf of your mother with the intention of making up for her missed fasts is valid and you will be rewarded for it. This is a form of filial piety towards her. As for combining the intention of making up for your mother's missed fasts with a voluntary fast, there is a difference of opinion among scholars. It is better for the intention to be singular for fulfilling the debt on behalf of your mother.