What are you trying to figure out, how co-op works, or how saving works?
In the single player game you control 2 main characters, up to 2 mercenaries and up to 2 summons. Anyone joining your game in co-op mode will take over the non-lead main character and then the mercenaries (for up to 4 players).
For saving, consider the situation where you are writing a book with someone else, where you get together, brainstorm and write down everything said (with no edits or backtracking). After a couple sessions, you friend makes a copy of the book so far, goes home and writes another chapter. In your next session together, you have the story at 2 different points; you can open your book and start at that point (either ignoring or recreating your friend's work), or your friend can open his book, and you can go from there. Or you can each stick with your own book, and write the rest of the (diverging) stories yourselves.
Thanks for clarifying that in a comparison, I can understand those ;p
