1) build an action that fires on the keyup action of the first field
2) Get the value
3) get the closes repeater
repeater = e.$el.closest('.acf-repeater')
4) Then locate the sibling field
repeater.find('[data-key="field_12345678"] .acf-input input')
5) adjust the value an insert it into the other field
The other option is to do it when the post is saved. See acf/save_post https://www.advancedcustomfields.com/resources/acfsave_post/. Get the values from the repeater and use https://www.advancedcustomfields.com/resources/update_row/ to update the other field.
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