There is a reason if the writer can think of one. That's the job of a writer, to plot things out for a desire goal in mind. This is a common feature that happens in Pokemon Adventures among nearly all the companion groups, often trying to reach two or more locations when what little time they have demands it. In fact, there are discussions over how the SwSh arc could be handled and one idea I suggested based on this common occurrence is that manga counterparts to Victor and Gloria for whatever reason could split their paths by Hammerlocke City. After that, they could battle the 5th and 6th gym leaders and when they go to the other cities neighboring Hammerlocke, they would battle the other 5th and 6th Gym leader again only this time it would not be Bea & Melony, but Allister & Gordie due to a change in Gym Leaders. Going back to what you said previously, this would not necessarily be repetitive but open up a whole new host of story ideas for the anime.
As for past arcs. Yes they have. The Gen 1 arc features three protagonists who each share a same goal, but perhaps your thinking of something more like as a traveling group right? Then look no further than the XY arc. And even if you say that doesn't count because the goal is not a gym challenge, it's still proof that having the same goal among companions isn't as repetitive as one thinks. So let's go back to the original topic, can two characters having the same goal be interesting or maybe in yours and the other person's case, can two characters who travel together having the same goal be interesting and based on everything I've said in the past, yes.
Never say never. If someone said that Ash would finally be sharing the spotlight with not simply just a main character but a fellow leading main character where they travel across the eight regions years ago, one would think there's no reason for Ash to travel that way, just as you said there's no reason for companions to split paths.
