MarqFJA87 said:
1) The two words are synonyms.
But they really aren't? A frown can be a look of discomfort or frustration or sadness or worry. It's an exceptionally broad term. A scowl is more specific; it refers to a hostile expression, one that directs displeasure towards someone or something.
2) The two tags' definitions on their respective wiki articles are effectively identical.
3) frown's article explicitly references "scowl" as an alternate name.
A wiki rewrite may be in order, but I'm not sure it would help much. Facial expressions are notoriously hard to describe in few words. You either know what they look like or you don't.
4) frown eclipses scowl in sheer number of posts.
Unsurprising, considering how bad a lot of users seem to be at recognizing and tagging emotions and expressions. I'm actually quite amazed by the number of posts that are tagged frown, because I see faces that would fall under the frown umbrella all the time that either get tagged expressionless or are simply left untagged.
There's no harm in leaving this tag alone. Everyone who can read faces well enough to tell the difference between a frown and a scowl will continue to use both tags differently, and anyone who can't read faces isn't being affected by the existence of scowl anyway.