You could do something like this https://www.advancedcustomfields.com/resources/dynamically-populate-a-select-fields-choices/ but your select value would have to have some why to know if the selection is a post of category.
For example you could use
label: Post Title
label: Term Name
Then loop over the selected values to display, parse the value to figure out if it for a term or post and then do the appropriate thing for that object type.
This would cause a significant loss in performance.
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