Is it permissible to pray a supererogatory prayer with two intentions, such as the intention of the Sunnah of 'Ishaa' and the intention of the Witr Sunnah, according to the Shafi'i school of thought?

The Sunnah prayer of 'Isha' and the Witr prayer are not valid with a single intention, because each of them is an independent act of worship intended for its own sake. It is not permissible to pray an independent supererogatory prayer intended for its own sake along with another similar supererogatory prayer with a single intention.