Is it permissible for someone performing a supererogatory prayer behind someone performing a fard (obligatory) prayer to follow him in the last two rak'ahs of his fard prayer, and when should he say the tasleem?

It is preferable to pray the pre-canonical (Sunnah) prayer individually, though it is permissible to pray it in congregation occasionally. The emulation of a supererogatory worshipper by one performing a fard (obligatory) prayer is permissible by the agreement of jurists. Therefore, if one prays the regular (ratibah) supererogatory prayer behind someone performing a fard prayer, it is permissible for him to do so. This applies whether he prays four rak'ahs continuously with him, or two rak'ahs and then emulates him in the last two rak'ahs and makes taslim with him, or emulates him in the first two rak'ahs, then sits and completes his tashahhud, separates from him and makes taslim, or waits for him to make taslim and makes taslim with him.