"Either", like "each", is singular, and you should therefore use the singular form.
Be considered wrong by who? Your friend on the bus? Your teacher in English class? A magazine editor on the internet?So, would the following sentence be considered wrong?
I see you are skillfully using the passive to avoid specifying who you mean!So, would the following sentence be considered wrong?
If either of you have seen him today, please let me know as soon as possible. Thanks.
If I hadn't checked Ngrams, my reply would match post #10. But if people say it (even if I don't) it is correct.Since "either of them" means "either one of them", I don't see how a plural verb can be considered correct.