Unas has tied the cords of the shem-shem plant, Unas has united the heavens, Unas rules over the lands, the South and the North. as the gods of long ago. Unas has built a divine city as it should be, Unas is the third at his accession.

Naydler explains:
"In a possible reference to a baptismal ceremony associated with his accession, the king is described as 'the third at his accession.' As a third, he would be between Horus and Seth (or Horus and Thoth), who would be standing on either side of him and would pour baptismal water over him. The position of the king between the dual gods, receiving blessings from both, symbolizes his union of their opposing natures within himself."(pages 305-306)

The baptismal water he refers to shows in a much later relief "Horus and Thoth purifying Ptolemy XIII at the temple of Kom Ombo". In both Unas and Ramses III, the deities have their hands at the pharoah's crown.

I wondered at this change to Horus and Thoth, rather than Horus and Set, and thought it was due to changing attitudes towards Set. But Wilkinson shows this is not so, for even when Set is not shown, he is still understood to be there:

Giving examples of when 'two' actually represents 'four', "in a classic study of the royal purification ritual, Sir Alan Gardiner showed that the two gods usually depicted performing the act of lustration - Horus and Thoth (ill. 124) - actually represented the four gods of the cardinal points Horus, Seth, Thoth, and Anti who transferred to the king a portion of their power as the divinities of the four quarters of the world. Private representations of funerary purifications (which were symbolically parallel) actually show four priests performing the rite, but the royal depictions of this ritual almost always depict only two of the deities, perhaps for purposes of symmetry and representional balance. Whatever the reason, once again we see two representing four and thereby carrying the connotation of the extended number, though the use of the two deities Horus and Thoth (paralleling the common use of Horus and Seth) may also have connoted the dualism of Upper and Lower Egypt." (from _Symbol and Magic in Egyptian Art_, by Richard H. Wilkinson, page 139)

However, it does seem in the Middle Kingdom and New Kingdom, it is Set and Horus that are more often depicted.

Amenemhat I, the first king of the 12th Dynasty demonstrates this on his lintel from his pyramid temple at el-Lisht:

A search of Karl Richard Lepsius' Denkmäler aus Aegypten und Aethiopien, turns up an image 18th dynasty's last pharaoh being blessed by Set and Horus:

Horemheb played an interesting role in Egypt's history. Not only did he do much to clean up the mess Akhenaten had made, he initiated the nineteenth dynasty when he appointed Paramessu to be prince regent and vizier who then assumed the throne as Ramesses I when Horemheb died.

"The role assigned to Paramessu once more reveals Horemheb's preoccupation with the military situation in Egypt's northern terrorities. Paramessu's family came from Avaris, the former capitol of the Hyksos, and the role of its local god Seth, who had retained strong connections with that of Horus of Hutnesu in Horemheb's career. In the light of this it is interesting to observe that Horemheb built a temple for Seth at Avaris. The Ramessid royal family considered the god Seth to be their royal ancestor, and a fragment of an obelisk (originally from Heliopolis), recently discovered on the seabed off the coast of Alexandria, shows Sety I as a sphinx with the head of the Seth-animal offering to Ra-Atum." (_The Oxford History of Ancient Egypt_, edited by Ian Shaw, page 294)

Seti I, who reigned not very long after Horemheb, has scenes in which he is being purified thusly by Set and Horus.

Merenptah, who ruled directly after Ramesses II, who has a blessing scene at Abu Simbel has a Heb-Sed festival lintel as well: