What is the ruling on performing supererogatory prayers when combining two prayers, and does the Sunnah of Dhuhr remain after the 'Asr prayer that is combined with Dhuhr?

"Whoever intends to combine two prayers should not observe the regular Sunnah prayer between them. Rather, it is permissible for him to pray the Dhuhr Sunnah after completing the combined prayer. Al-Mardawi said in 'Al-Insaf': 'He prays the Dhuhr Sunnah after the Asr prayer without dislike, and this was stated by most of the companions.'"