You can try to build something in JavaScript to do it on the page. That would be too much code to post here but I can give you the general steps. You need to
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.
