Magic exists, so no life will be detected subsequent to the smaller party’s “demise”; and with the villain announcing it, well, it’s pretty much a given under the particular circumstances. The thought that the smaller party hasn’t fully crossed over won’t occur to them. Main party’s going to suspect traitors in their ranks.
All the plans the hero party (combined) had at the time of the separation were falling apart. So, main party is going to persevere until they get wind of what the villain is after and try to stop it. Yes, the smaller party will be playing much harder, going straight for the endgame, even getting into squabbles between themselves, at times, because of how hard they’re pressing.
This story has four MCs, two go to the small party (& presumed dead), whereas the other two regroup with the other members of the general organization the “main party”, which has some level of disbelief/disarray because the deaths weren’t what they were hoping for.
And in the smut version, small party’s going to be naked