There isn’t any real way to do what you’re trying to do. Each ACF field only saves a single value. From the user entering the information perspective, that “units” drop down would need to be an entirely different ACF field.
Rather than using a single field I would use 2 fields and use the width settings to put them side by side rather then attempt to use a single field.
You could probably do this if you created a new field type https://www.advancedcustomfields.com/resources/creating-a-new-field-type/
