Just to cover all the bases (yet another baseball idiom), twice more Paul cites "the two will become one flesh" from Gen. 2:24. In both I Cor. 6:16 and Eph. 5:31 the immediate context is sexual immorality, but Paul uses the phrase in reference to the believer’s being joined with Christ. The NT idea of mystical union with Christ doesn’t really hinge on this analogy from the creation, so unless someone else has something to say about these two Pauline passages we’ll move on.