Danbooru's long-standing practice is always to tag a futanari character as female, and adding a female character to that is, of course, 2girls. I think newhalf gets treated the same. You could argue it unnecessary with other tags like futa_with_female, but the "#girls" and "#boys" tags are more about counting characters and the expectation of what you want to see when you perform the search.
Speaking of which, search supports minus-keywords so searching "2girls -futanari" should produce exactly that.
