Is it incumbent upon the traveler whose ablution was nullified between the Dhuhr and Asr prayers after performing the Dhuhr prayer to repeat both prayers?

A prayer does not become invalidated after its completion if it has met its conditions and pillars. The invalidation of one prayer does not affect the invalidation of another, even if they are combined. So, if ablution is nullified during the second prayer of a combined set, only that prayer becomes invalid, and the first prayer that preceded it does not. In this case, one must renew ablution and restart the second prayer. It is obligatory for anyone intending to combine prayers to promptly renew their ablution if it is nullified, because continuous performance (muwalat) is a condition for combining prayers by way of taqdim (advancing the later prayer) according to the majority of scholars.