Best way to look at it is this Ammokk:
One person owns everything. They own the game, they can remove the other person any time they fancy... it's identical to owning the sega genesis/megadrive and being player ONE.
The other person comes into your game at any point, and leaves at any point too. They can join in at character creation and make one of the two starters (you only ever start with 2 chars, you don't make your own pair of two so you never begin with 4).
While they control the character you have no access to it. You can send stuff to it, but you can't take stuff from it. On top of this, the party hires 2 henchies that you can share (you can reassign them willy nilly, meaning if you're busy stealing things, you can give both to the other person to fight with).
When you save you save the game, you don't save the participants. If the person is still with you when you load it up (let's say you saved before a dialogue choice you didn't like) then things wont change. But if you start it up the next day they'll simply rejoin your game and you can continue on your merry way.
On top of this, you could make a new game, play a bit, hit level 4 and then they could join you (say you had the day off, they were working) and take over a character.
So to be clear - it's a "drop in, play a bit, leave if they fancy" game... unless you remember you have free will, and you can simply wait to play the save just with them!!! (tip: if you play more than them, remember after each session to save it to a proper save slot, as the quicksave/autosaves will overwrite eventually.)