[quote=@Sombrero]
<Snipped quote by Shorticus>

I guess so. My problem with groups, though, is the classic issue of different people wanting to go different ways, which could be solved by making more groups and threads, (There's so many things to go and do, one group with one goal wouldn't cover it very well) but... I guess this all really revolves around my deepest fears that someone will do something which would intersect someone else's stream of continuity a few events before the time they're at currently, and then I'd either have to retcon everything that happened to the players in the future, throw off the other players' plans in the past with some inexplicable obstacle, or drag both of them across the planes of spacetime so that the event could occur properly.
[/quote]

Yeah, that's actually a really good point. I don't have any GREAT advice for that situation, but here's something I've been told by other tabletop GMs a lot:

When you slip up, or when the story suddenly produces a bump, work it into the story.

The classic example I was told once was a DM who had a vampire walking around freely in the sunlight. One of the players said, "Hey, what the Heck? We know that guy's a vampire. How can he walk around in the sun?"

To which the DM, who did NOT intend to make that blunder, replied: "That's a very good question. How COULD a vampire survive in the sunlight?"

This became a plot in which the vampire was actually wearing an amulet that allowed it, blah blah blah. Point being: most problems that occur can be worked out by a little "Well, yeah, that WOULD create a problem. I wonder how it still happened, hm? Hm?"

In your situation... Let's say Group A talks to a duke, but Group B (who is a week behind) comes in and kills the duke. Well, crap. What do we do with the duke?

BOOM. Doppleganger.

Not saying this will work for every situation, but you get the idea. You can do some sneaky stuff, even if you have to rework some things.