Is the prayer performed once with the intention of both Asr and Maghrib when making them up after the time for Maghrib has begun and before the Maghrib prayer, or is it performed twice?

If you find yourself in this situation, perform the call to prayer (adhan (definition)) for Asr and pray it, then perform the call to prayer for Maghrib and pray it. Delaying the prayer beyond its appointed time without a valid excuse (such as forgetfulness or sleep) is not permissible and contravenes the words of Allah Almighty: (Indeed, prayer has been decreed upon the believers a decree of specified times) and His words: (So woe to those who pray, [But] who are heedless of their prayer). Furthermore, it is understood from your question that you are neglecting congregational prayer. Be diligent in observing prayer at its appointed time and in congregation.