Well, what you’re looking for is not completely possible with ACF.
If you’re okay with then field updating when the post is updated then you can use this filter http://www.advancedcustomfields.com/resources/acfload_value/ and you can adjust the value on the second field by the value of the first.
Having a field that is not editable is another story entirely. ACF doesn’t currently have a way to set a field as readonly. I would suggest that you take a look at this add on https://wordpress.org/plugins/acf-enhanced-message-field/
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