In single player you control both of the main characters yourself, as well as any henchmen hired (one per main character at a time allowed in the party; others can be sent to wait in your homestead on the shelter plane at the end of time). You can switch back and forth between any character, and stay in a group or split up your party.
In terms of conversations, the non-lead character can be set to always agree, answer as a specific personality or answer randomly, or you can choose the dialog options for both characters (if both are close enough to be involved).
Up to 4 players are supported for multi-player in mods. Larian wanted to focus on the two main characters in the main campaign, so didn't write a ton a dialog for them, so that a person controlling them could participate in conversations. The editor, however, supports dialog for henchmen. Meeting the henchmen stretch goal means they will have dialog, but not to an extent a person could roleplay them as they wished.
