I kind of feel that both of these should be merged with synonyms. I guess the word "synonym" may tend to connote a single word, but it can also refer to a phrase or expression. In any case, users can make it clear which kind of synonyms they're looking for by adding the tags single-word-requests or phrase-requests.
The only possible downside I see to this is that it would make it harder for people who want to "favorite" only questions requesting alternative words, and not other questions about synonyms such as "what are the differences between these synonyms." There are only 2 followers of the tag "alternative," however.