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Tolkien dealt with exactly the same situation in The Lord of the Rings, starting with the breaking of the Fellowship. For example, we have simultaneously Merry and Pippin being carried by orcs; Ara...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/43642 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/43642 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Tolkien dealt with exactly the same situation in _The Lord of the Rings_, starting with the breaking of the Fellowship. For example, we have simultaneously Merry and Pippin being carried by orcs; Aragorn, Legolas and Gimli chasing same orcs; Frodo and Sam getting lost in the Emyn Muil. The way Tolkien lets us know how events relate to each other on the time axis is by having "landmarks" that are "seen" by two or more groups. That is, an event occurs, two parties see it, so we know event X in the first group's story occurs at the same time as event Y in second group's story. A nazgul is seen by Pippin. Same nazgul, many chapters later, is seen by Frodo and Sam. We know that those two occur at the same time, since it's the same nazgul. Your situation is complicated by the fact that you start with two separate groups, while Tolkien starts with the party together, and then splits it up within the narrative. Nonetheless, is there any event that could be shared by both your section 1 and section 2? That would be a very clear indication to the reader that those occur at the same time. Alternatively, you say your sections 1 and 2 do not occur in the same world. Does time flow the same way in both? Does simultaneity have meaning between worlds? Reading of events that are very separate, with no shared characters or locations, I would not automatically assume that event II occurs after event I. If there is some cause-and-effect relationship between them, then yes, I would assume effect happened after cause. Otherwise, I would hold my judgement on how the two events correlate on the time axis until the plotlines merge, or I'm given some other form of explicit information by the author.