By the same token a merchant forgets he hates three of them, he might just like the one he does like, enough to overlook their company. There would be some reason he does like the other guy, and as far as SP goes, the game already assumes a party split that's more convenient(i.e.: you are alone for the purpose of conversations that require you to be alone, even if everyone else is standing right next to you).
As far as group present and closest first goes, that's an issue much more easily solved by allowing you to switch who's talking.
You have to keep in mind that it would make Persuasion checks impossible to fail if it just stacked, cause you could simply put a point in Persuasion on everyone, and have up to 4 times what's in theory possible right now.
Even doing just double the current curve would be silly, and unlike Lore and Luck, which do not explicitly benefit from going above the curve, it would trivialize the Persuasion challenges.
