Why Lord Shiva has two scorpions on his ears?

Serpents Kambala and Asvatara are Lord Shiva's earrings 

Kedarakhanda- Maheswarakhanda of Skanda Purana mentions names of all serpents which are present on Lord Shiva as ornaments.

Narada goes to Kailasa and sees Lord Shiva as follows:

Similarly, Sambhu, the lord of the three worlds, was seen by Narada, as being served by Suras and Asuras. The lord had the luster of pure gold. 

34-35. His lotus-like feet were served by Sankha, the excellent serpent. He was served by the following serpents: Dhrtarastra, Takşaka in particular, the great serpent Padma and Šeșa in particular. He was continuously served by other excellent serpents. Vasuki had become a necklace of great luster and was retained in the neck. 

Kambala and Asvatara were his perpetual ear-rings. Other excellent serpents were ensconced at the root of his matted hair. 

37-44. He was covered by serpents of many colors and types, e.g. Taksaka, Kulika, Sankha, Dhrtarastra of great luster, Padma, Dambha, Sudambha, Karāla, Bhīşaņa-these, and many other serpents formed parts of Hara who was the most worthy of being worshipped in the three worlds. 

These two Serpents live in Sutala loka. Sutala loka is one of the nether worlds below the earth. Seven Nether lokas are Atala, Vitala, Sutala, Talaatala, Rasatala, Mahaatala and Paatala. 

Chapter 20, Anusanga of Brahmanda Purana says

20b-25a. In the nether-world Sutala, O Brähmanas, are the abodes of the following, viz. the city of the first lord of the Daityas and Räksasas i.e. Of Mahäjambha, the palace of Hayagriva, Krsna and Nikumbha, the city of the Daitya named Sankha and that of Gomukha, the abode of the demons Nila, Megha and Kathana, the abode of Kukupäda and that of Mahosnisa, the city of Kambala the serpent and that of Asvatara and the city of Taksaka the noble-souled son of Kadru. Thus there are, O Brähmana, thousands of cities belonging to the serpents, Dänavas and the Räkshasas in the nether-world of pale white coloured soil.

If you are wondering how could those serpents live at two places simultaneously, divine amsas of them could stay with Shiva forever while physically they could live in Sutala loka.