Hi @jmb
You should use the ‘acf/update_value’ filter. Docs here: http://www.advancedcustomfields.com/resources/filters/acfupdate_value/
You should target the second field, not the first. I’m guessing this is ‘last_name’.
In your filter function, you can get the last_name value from $value (a parameter of the function), and you can get the ‘first_name’ via a standard get_field function.
Hope that helps.
Thanks
E
