How do Campaign Playlists function?

So I understand how the playlists work in terms of it being a set of levels that you can play through without having to manually select them. What I want to know is how many can I actively play at one time? If I make progress on say Halo CE Legendary Campaign then stop and go to Halo 3 Normal Campaign is my progress saved for both and can I go back and continue either at any time.

The main reason I’m curious is because I’m doing Legendary Campaign co-op with a few different people. Mostly for their sake since I’ve got the achievements. One guy I’m working on Halo 2 with and the other wants to start up Halo CE. Now say Guy #2 and I progress through Halo CE fast enough that Guy #1 and I aren’t finished with Halo 2 if I start Halo 2 with Guy #2 will it overwrite my progress with Guy #1?

I’m mostly curious about running multiple games with multiple people and not having to start from the first level on each every time.

Thanks for any bit of clarification anyone can shed on this.

I personally think that playlists are broken. I like the concept and creates replay missions not boring. Anyway, the way they are now, if you start a playlist and decide to start another after playing a few levels, the progress will be saved… but you will still have to start the playlist over. Also, theres a bug that if you start a mission and dont finish it, it will make you play the next in the list and you will have to play the whole thing again to get to that level and complete it. Broken