Considering there will only be three opportunities for feats in the game, dual wield for a dagger is a non starter. If those are the rules and Larian wants to stick by them, I won't complain, but I am not going to withdraw the suggestion. The rapier is not a heavy weapon, classifying it as such to avoid someone dual wielding them makes s sort of sense, but the weapon would come to be specifically designed for use with an offhanded buckler or dagger for defense in a few short decades after its initial design (disregarding medieval references to rapiers which described any slashing sword with a guard and lacking a cruciform hilt). Their average weight somewhere between two and three pounds but when balanced well they feel all but weightless.
Its up to the devs what they want to do, but I know what course of action I would encourage
