This feature is not yet directly available. You can vote for it here: https://trello.com/c/I8wjR1yf/291-merge-2-actions-words.
Meanwhile, here is a workaround to merge two action words. First, in the action work that you want remove, call the action word that you want keep in the definition. Next, click on the ‘Inline’ button.
To finish, in the modal, click on the “Inline” button and next click on “Delete action word”.