They aren't an "official" couple per say, but at this point everyone just treats them as such and neither of them deny it. For example back when the arc first started and Revy visited Feng at her place after she kissed Rock, Feng told Revy she thought Revy was there for putting hands on Revy's man to which Revy said if she was there for that that Feng would be dead already. I think that's a pretty clear sign.