How are the supererogatory prayers (nafl) for Maghrib and Isha performed when they are combined due to rain or bad weather conditions? Are they combined into one supererogatory prayer, or does each prayer have a separate supererogatory prayer?

The permissibility of combining prayers (Maghrib and Isha, or Dhuhr and Asr) due to rain or similar circumstances is agreed upon by the four madhhabs and others. A condition for combining is sequence (tataabu'), meaning no supererogatory (nafl) prayer should be performed between the two combined prayers; otherwise, the combination becomes invalid. The supererogatory prayer should be performed after completing the combined prayers, individually and with a separate intention. It is preferable for it to be performed at home, though it is permissible to perform it in the mosque individually or in congregation.