ACF stores URL fields and Page Link fields in two different ways. A page link field stores the post ID and the URL field stores the actual URL.
This has actually been suggested before. What we need is for someone that really needs this to build a new field type for ACF that includes a radio button to choose the type of link and somehow combines all the other controls of the two built in field types. http://www.advancedcustomfields.com/resources/creating-a-new-field-type/
Other than than the only way to accomplish this is to add a radio field that allows the user to select the type of link and then have two conditional fields, one for a a url and one for a page link, that are shown based on the radio selection.
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