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A/B or Split testing in this context probably means putting up two different listings for an item that will be shown or described in two different ways. The intention is to see which produces the best results. While it is fine to describe two different items using two different templates, title arrangements, and pricing structures, they must indeed be two different items.
eBay does not allow duplicate listings for identical items although they do have one exception -- one item can be listed as Fixed Price and the other can be listed as Auction, so you can play around with the details in those two formats if you have more than one identical item.
https://www.ebay.com/help/policies/listing-policies/duplicate-listings-policy?id=4255
There can not be two separate listings for only one available item. The closest that eBay provides is the ability to list an item in two different categories simultaneously for an extra insertion fee. It is still only one item, but the extra category expands the search possibilities.
Alternatively, if you have two very similar, but not identical, items, you can list each one with a different focus as part of your split test. The most common split tests are to list with free shipping versus listing with calculated shipping and to list Fixed Price versus Auction.
