Here is what I've come up with so far:
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In that year, I worked for two different companies, but not at the same time. (This version doesn't feel right)
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In that year, I worked for two different companies in succession.
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In that year, I worked for two different companies consecutively.
So for example, I had one job, then quit it a few months later to work for another company.
Best Answer
Even simpler:
Using "then" makes it clear that you are talking about a sequence of events.