I can't imagine why you'd leave both Karlach and Astarion out of your party, unless your version of Tav, the hero of Baldur's Gate 3, is some kind of obscene rogue/barbarian multiclass who has both their specialties covered. That's what happened at GenCon's live Dungeons & Dragons show, however, when voice actors Samantha Béart and Neil Newbon explored what their characters get up to when they wander off on their own, live on stage with Dungeon Master Aabria Iyengar and fellow players Anjali Bhimani (DesiQuest) and Brennan Lee Mulligan (Dimension 20).
Their adventure begins at a dive bar so low it's in the actual Underdark, but I won't spoil where it goes from there. Suffice to say it's an extremely silly good time, and the players get to take a couple of pot-shots at Gale and Lae'zel while they're not around. This is for sure the kind of shenanigans the NPCs get up to when I leave them behind, and that's canon.
