The trouble with not sharing social skills is that one character will end up doing all the talking (and buying/selling) while the other sits and watches. This isn't much of an issue in single player, where the logistics of it don't really matter, but it's not particularly multiplayer-friendly if one player gets stuck twiddling their thumbs all the time because the other one has the social-skilled character.
Nor is it a good idea to expect both characters to crank their social skills because - apart from putting a serious damper on character concepts for your two-person party - it's also largely redundant in single player, which means that it ends up as a 'multiplayer tax'.
Either one of these is particularly bad given the design intent of drop-in multiplayer.
