What prevents combining more than one Sunnah in two rak'ahs, while combining is not permissible in fasting? And is it permissible to combine a Fard (obligatory act) with a Tatavvu' (supererogatory act)?

There is no harm in combining two acts of worship with a single intention if they overlap, such as the ghusl for Friday and for Janabah (ritual impurity), or if one of them is intended for its own sake, like the Tahiyyat al-Masjid (mosque greeting prayer) with an obligatory or supererogatory prayer. However, it is not permissible to combine two acts of worship that are each intended for their own sake, such as the Maghrib prayer with its Ratibah (supererogatory prayer accompanying it).

It is permissible to combine the Tahiyyat al-Masjid with the Sunnah of Wudu (ablution) and the Sunnah prayers performed before an obligatory prayer because the objective is to perform a prayer, and that has been achieved.

It is not legislated to combine the intention of fasting Ashura with the intention of fasting on Thursday, as both are supererogatory fasts intended for their own sake.

As for combining a missed obligatory fast (Qada') with six days of Shawwal, it is not permissible, because fasting the six days of Shawwal is a supererogatory fast intended for its own sake.