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Jehovah's Witnesses agree that Ephesians 4:8 refers to Jesus and that Psalm 68:18 refers to Jehovah God (Yahweh). They also agree that it equates Jesus' actions with God's actions, but they disagree that it equates Jesus with God.
Jehovah's Witnesses believe Jesus is God's Son and God's representative, and as God's representative, it's Jesus' responsibility to imitate God. Ephesians 5:1 tells us to "become imitators of God, as beloved children." Jesus, being God's Son, gave a perfect example for the rest of God's children in imitating God.
Those favored with assignments of service by God’s undeserved kindness are also called “gifts in men,” and Jesus Christ, as God’s representative and head of the congregation, has given these to the congregation in order that its members individually might be built up and attain maturity. (Eph 4:8, 11, 12)
—Insight, Vol. 1, pg. 937 "Gifts of Service and 'Gifts in Men.'"
To confirm that Paul didn't equate Jesus with God, Jehovah's Witnesses could cite the verses directly above at Ephesians 4:4-6, where it says in the ESV:
4 There is one body and one Spirit—just as you were called to the one hope that belongs to your call— 5 one Lord, one faith, one baptism, 6 one God and Father of all, who is over all and through all and in all.
This verse says the Father is "God and Father of all." If the Father is the Father of the Son, then according to this verse that would mean the Father is also the God of the Son. Therefore, the Son isn't God. He has a god: the same God and Father everyone else has.