It is not required to use both. However when using portfolio, I would probably be more inclined to use the original estimate field when the issue has no estimates at all yet. This can be useful later on when this issue is then seen by your teams working those issues. Those team members might adjust the estimate on the issue, or log work against it. However the original estimate is not expected to change once the issue has started logging work against it, or it exists in an active sprint. But by using the original estimate field, you can then compare against the other estimate field to see how close you were.
I hope this helps, but in case it does not, perhaps check out this recent blog post about using estimates for Portfolio: Estimation best practices with Portfolio for Jira
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