That was even worse, for once I hated a Lucario(My #1 fav) being around. There was no reason for both it and a Piplup to be around.... if they were with the same character, then sure that's fine! But two non-Kanto natives with Ash, neither are related... and they both happen to be from Sinnoh. Yet Sinnoh remakes didn't happen(Cause Kanto needed it's 5th turn *sigh* )
In a way I was glad that they decided to go with the non-obvious route by showcasing two popular Shinou Pokemon in that movie and not just a bunch of Kanto Pokemon.
Yuyama has explained that Makoto and Soji have the partner Pokemon they have because they're two of the most popular Pokemon out there, and that it wasn't until afterwards that they realized both Pokemon happened to come from the same gen. So that was a pure coincidence.