What is the ruling of Tayammum (dry ablution) for whoever combines Thuhr and ‘Asr prayers when travelling?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

It is permissible for a person who is travelling to perform Tayammum (dry ablution) in order to perform Thuhr and ‘Asr prayers if he did not find water. It is also permissible for a person who has performed Tayammum (dry ablution) to perform more than one prayer with the same Tayammum, whether the obligatory prayers or the supererogatory ones; however, there is a difference of opinion among the scholars on this issue.  

Allaah Knows best.