What you are ignoring is that the 2 characters in this world are static story components.
They are not "generic" characters that are interchangeable like in NWN. BOTH characters exist in the game all the time. The second character doesn't only appear when a second human joins. The host controls both until a friend joins. You can't just wipe the hosts character to overwrite it with a random character of different class/inventory/personality. What happens to the character the host has been playing when this new person joins? They just lose them forever? If they come back when the friend leaves, what about all the plot choices the friend made? Have they permanently changed the host's version of those characters?
The mechanics of this game are completely different than NWN or any table top campaign designed around unknown generic characters. You cannot simply swap characters in and out as you say.
