The thing is, we know only a few elements about Merovech as a founding figure of Merovingians, and most of it is mixed up with fantastic features such as a monstrous origin. When mentioned in Frankish history, it is always in relation to genealogies which are themselves probably disordered : Merovech as father of Childeric is only one of the available filiations even if it's the more plausible.
Arguably, for Christian Settipani, the disordered list of Salian kings might be explained by a struggle of succession from which Merovech and his lineage would have been partly ousted (as it might have happened for Childeric later on and forced him to exile).

This struggle, and the relation to Attila, would be contextualized by a comment made by Priscus, an historian who had direct knowledge of Huns due to its diplomatic activity, about the Catalunian Plains (fragment 20)

Attila justified his war against Franks by the death of one of their kings and the dispute between his sons for supremacy. The elder decided to ally Attilla, as the younger joined Aetius. We met him when he came to Rome. His face was still covered in fluff and his blond hairs was so long he braided them. Aetius made him his adoptive son and, as the emperor did, covered him with gifts and sent him back as a friend and ally.

What do we learn there? That one of the Frankish kings died and that his sons fought over the succession, the younger being barely out of adolescence.
At this point, it should be pointed out that Salians were only one of the petty-kingdoms in obedience to Rome along with the Rhenish Frankish ensemble : there's nothing certain about Merovech being even involved there. Franks being somewhat of secondary interest in most of the first half of the Vth century as far Romans were concerned, details are lacking.
We know that Salian Franks difficultly expanded over Belgica in the decades before 451, led by their petty-king Clodion as recorded by by both Sidonius Appolinaris and Gregorius of Tours.

The latter accounted Clodion as the father of Merovech, so if Priscus' text is describing a succession crisis among Salians, Clodion still being active in 447/448 when Majorian defeated him means that he would have been the deceased king and Merovech involved as a possible son or heir and potentially the younger son thus described.
On the other hand, and again Frankish royal lists are made tardily and messily, Gregorius accounts Childeric being already king in the late 450's. Merovech would have been only king for some years? It could mean that Clodion was dead already by the 450's, succeeded by Merovech and that Childeric would be the adoptive son of Aetius, which would be fitting the reinforcement of the Roman-Frankish alliance under Childeric.
It's possible, in either cases, that the "elder son" isn't otherwise known or by a name thrown in later royal lists.

Maybe this have nothing to do with Salians. Merovech or Childeric are not recorded having fought the Huns (which doesn't mean they didn't) : it's quite possible that Rhenish Franks were rather thus described, Salians being "naturally" joining Aetius as federated as Jordanes accounts them, fighting the Gepids who were under Hunnic hegemony.

In the state of our historical knowledge, it's impossible to assert what happened there : sources on early Frankish history are scarce, and the Merovingian lines by all means was an obscure dynasty before Clovis warlorded his way in Gaul. Many specialists on Merovingians carefully avoid taking sides there.
Thus this seemingly conflicting affirmation you're looking at : note that neither author is saying Merovech and Childeric fought on different sides, but attempting separately from each other to identify the "elder" and the "younger" son as respectively Childeric and Merovech.
Likewise, there is a lot of conflicting and contradicting interpretations about how involved Frankish petty-kingdoms were in the fight against Huns even if the end result of a stronger (or renewed) alliance with Romans, especially with Salians, is less disputable.