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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is permissible to make Tayammum when one prays Thuhr and ‘Asr together while traveling and there is no water available. It is also permissible for one to pray more than a prayer, whether obligatory or voluntary, with only one Tayammum, noting that scholars held different views in this respect.
Allaah Knows best.