From what I'm reading, some theories, like the Many-Worlds or Heisenberg Uncertaintly Principle, hinge on some sort of event that actually creates the parallel universe, such as a scientist simply observing the state of a quantum object (whether in wave or particle form, wave vibration, etc...) or even by making the decision to not make a determination at all. Such an event can cause the split of the universe to accomodate for all possible outcomes of the observation. What a powerful thing to consider: that even the THOUGHT of multiple outcomes could create those multiple outcomes somewhere. If we'd ever find this to be true, the human mind should be considered the greatest object ever created. Sort of makes you reconsider trying to discuss Schroedinger's cat (Google it). Using this information to my question, my question still remains unanswered; we could still share history, there could be no history prior to the event for the alternate world, or they could create a history from shared thought-pooling once it is established. If we follow the theory of Dr. Kaku, which is called String Theory, it's much easier to believe that the universes have their own histories different from each other. His theory states that all objects in the universe, even on down to the smallest particles known currently as quarks, are made up of tinier building blocks that resemble rubber bands and what matter they create depends on these bands and how they vibrate. This creation happens across eleven separate dimensions. Because of how it's created, our universe is therefore separated by itself inside of a "bubble", which exists along side of the other parallel universes in their own "bubbles". It's theorized that they can interact with each other by sharing gravity, but that would create an explosion similar to what some scientists believe happened at the Big Bang. Probably not a good idea for this to happen. BUT, this string theory would then, as I said, give credence to the idea that any alternate worlds would have a unique history, some being just slightly out-of-phase from each other, but definitely unique to that "bubble".
These theories are just the most prominent; we've barely scratched the surface. Plenty of other scientists have their own ideas on if any of this is possible. The truth could be a combination of a couple or all of them, it could be one singular idea, or another possibility is that none of these can actually occur. Possibly by considering these in this blog, we've created some alternate realities of our own. Maybe some day we'll know...or in another reality, we already know. One thing I do know: I'm not anywhere even CLOSE to answering my own questions. But I think it's fun to keep trying!
If I haven't driven you nuts by this discussion, I invite you to join in. Any of your own theories? Questions that you think I or someone else that reads this may be able to answer? Post them here!