I'm writing here so it can be more visible.
What you are asking, imo requires logic and such I have a couple of suggestions in case you are using Script Runner:
1-Use Behaviours
You can create a custom behavior for the first field and set the 2nd one as required in case the 1st empty or else.
2-Use a Validator Post Function
Run script to check for the required values and logic.
Could you further explain the situation?
Like where, when you need to do this?
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Whether to apply for a project manager has a drop-down option with application and no application fields. When selecting the application field, the project manager field can be empty; when selecting the non-application field, the project manager field is required.
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