The following link, which adopts the same view, helps by providing a few other modern day examples of rhyming slang, e.g., “see ya later, alligator.”

http://members.aol.com/MorelandC/HaveOriginsData.htm

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As for the OTHER main explanation offered —

The Swiftian origin suggested by one (from a discussion forum entry) is intriguing, but I’m not sure the citation does much to demonstrate his use as the actual origin of our phrase. And there is no other specific evidence of anyone “picking it up from Swift”.

It seems more likely that Swift was simply playing with rhyming words himself, (or even possibly playing off an expression of this type that already existed, not coining it himself).

Note that the rhyme is simple enough it’s not difficult to see how different people might come up with something similar completely independently. ALSO, Swift’s use is ironic (they were not at all even!) while the common use of the phrase is NOT which tends to undercut the idea that its common use was based on Swift’s. (Reason –In a case like this, the more likely direction is generally that of someone taking an ordinary phrase and making an ironic play on it, not the other way round.)

Another observation — the line in Swift is slightly miscopied so may be confusing. It should read thus:

“Now we are even,” quoth Stephen, when he gave his wife six blows for one.

http://www.gutenberg.org/dirs/etext03/stlla10.txt

(see Letter 3, par 20)

In other words, there is a play on the rhyming words, BUT “even” and “Stephen” are not part of a single expression… and no one has explained how we supposedly got from Swift’s line to the current expression

: EVEN STEVEN — The term apparently stems from a character in Jonathan Swift’s “Journal to Stella” (1713): “‘Now we are even,’ quote Steven, when he gave his wife six blows to one.’ Stella was Swift’s name for Esther Johnson, and his ‘Journal’ letters to her described his daily life in London. Their relationship was a complicated one. Swift, 14 years Stella’s senior, taught her to read and write, loved her all his life and when he died was buried beside her, but the two lovers probably never married.” From the “Encyclopedia of Word and Phrase Origins” by Robert Hendrickson (Facts on File, New York, 1997).