I've played the multiplayer game only once, with a friend. I posted before about these things but will state the same again.
He did all the fighter/thief parts. He had a Fighter/Thief, A Paladin and a Ranger. I took care of all the magic, except the ranger ofcourse with an Illusionist/Cleric, a Conjurer and a Cleric of Helm. This way you both control an essential part of the game. We played without any mods and on the hardest difficulty. In my opinion this is the only way to play multiplayer with a full party with only 2 or more people. The only other version I could see happening is the version with six people and all 1 character but that would be very hard to accieve.
"certainty of death, small chance of succes, what are we waiting for?", Gimli, son of Gloin