Learn English – “Were that you who did that” vs. “was that you who did that”

it's contentious, but I would lean towards "was", since it is acting up the individual "either".

From The Cambridge History of the English Language: English in North America, p. 332, by John Algeo, via Google Books:

When thou, thee, and thine dropped out of the language in the early Colonial period on both sides of the Atlantic, speakers felt a need nonetheless to distinguish between singular and plural forms. The earliest attempt was simply to make verb agreement do the work: speakers would say you was for the singular and you were for the plural. Beginning in the eighteenth century, this sensible solution was met with heavy resistance from purists, and you was became heavily stigmatized by the end of the nineteenth century in America (though it has by no means dropped out of colloquial speech in the United States).