Button with different value and label / text
I think this might be a duplicate of a question I posted a few weeks ago for Drupal 8.

Original question: https://drupal.stackexchange.com/a/226494/57

Some more write up: http://kevinquillen.com/drupal/2017/01/28/adding-twig-template-suggestions-for-form-elements

Basically I wanted to target just one submit, hence that technique. Simply changing the #type to button is not enough in the Form API to get that result you're asking for.