Can the Imam lead the prayer for the two Rak'ahs of the Dhuhr supererogatory prayer, and then say the Taslim for the one being prayed for to complete his prayer?

If someone wishes to repeat a fard (obligatory) prayer as an act of charity on behalf of another who has not performed it, it is preferable that the one who has not performed the fard prayer be the Imam. This is to avoid the disagreement among scholars regarding a person performing a fard prayer behind someone performing a nafl (supererogatory) prayer. The hadith narrated by Abu Dawud and Al-Tirmidhi indicates the permissibility of joining someone who is praying alone, even if the one joining has already prayed in congregation.

It is also permissible for someone who has already performed the fard prayer to lead, based on the view of some scholars who consider the prayer of a person performing a fard prayer behind someone performing a nafl prayer to be valid. According to this view, you could lead someone performing the Dhuhr fard prayer while you are performing the two rak'ahs of the Dhuhr sunnah prayer, and then the follower completes his prayer after you have said the taslim. However, the cautious approach is for the Imam to be the one performing the fard prayer, to avoid the disagreement of those who do not consider the fard prayer behind a nafl performer to be valid.