I could see sharing Barter. Charisma shouldn't be shared, because the game is specifically programmed to allow for separate attitude modifiers for the two main characters. Luck shouldn't be shared, Leadership's bonus is provided for the team by design.
If you're talking about Charm/Intimidate/Reason, most of the choices have input from both characters. I've only seen one case where there's a single Charm/Intimidate/Reason dialogue check - with Roberts. Sure, in a situation like that, I guess giving control to the one with the highest skill could probably be done in a seamless and smooth... oh wait.
Hmmm... Actually that might be more complicated. First of all, not all situations are equal. Roberts has a situation bonus to Reason and a penalty to the other ones. So if you have one character with 2 in Charm, and the other guy has 1 in Reason, which one gets picked to do the talking? Second, there's no guarantee that the person who gets picked for the higher stat is going to pick the option that matches their high stat - they could pick something else.
