I just arrived at the cordon and done some of the missions there. It seems that the Stalkers really like me after helping them, and now I have as an option to join their faction.

I don't really get how the "big picture" of the faction wars works. I read this really informative thread on how the faction wars battles works, and the wiki page on the topic seems to shed a bit of light on the topic too, but I have some doubts on the topic:

· It seems that most of the fighting is through 1 faction vs 1 faction battles in different maps (i.e. Clear Sky only fighting renegades, Duty only Fighting Freedom), so I would not have a "Clear Sky versus Duty" battle going on in a vanilla game.
· So, can I join more than one faction on the game? Using my last example, joining Clear Sky and helping them in "their" battles, then joining Duty.
· If I can't, can I join a faction, do their quests, then leave for other (not opposing) faction?

Basically, I want to get the better of the game in one playthrough, but I'm afraid of blocking myself specific paths so early in the game for not having enough info.