In the excellent book edited by Burk Parsons, John Calvin: A Heart for Doctrine, Doxology & Doctrine (Lake Mary, FL: Reformation Trust, 2008), I find it amusing that two different writers who focus on two different topics end up with two conflicting views regarding the relation between justification and union [with Christ].
Philip Graham Ryken , focusing on the theme of Union with Christ, wrote Chapter 16 in the book entitled The Believer's Union with Christ. On page 196, he quoted from Calvin's Institutes 3.11.1 as follows:
Of all the blessings that come from being united to Christ, [John] Calvin emphasized two that are distinct yet inseparable: "By partaking of him, we principally receive a double grace: namely, that being reconciled to God through Christ's blameless, we may have in heaven instead of a Judge a gracious Father; and secondly, that sanctified by Christ's spirit we may cultivate blamelessness and purity of life." (Calvin, Institutes, 3.11.1) To put this in theological terms, by virtue of our union with Christ we receive the double benefit of justification and sanctification.
Ryken later writes on the same page:
As important as it is in its own right, Calvin's doctrine of justification finds it proper context in his doctrine of union with Christ. The blessing of justifying righteousness becomes ours when we are united to Christ by faith.
Our second author is Michael Horton. Focusing on the theme of justification, Horton wrote Chapter 17 in the book which is entitled The Principal Article of Salvation.
On pages 211-212, Horton wrote:
Regardless of whether union temporally preceded justification, Calvin is clear that the latter is the basis for the former: "Most people consider fellowship with Christ [Christi esse participem], and believing believing in Christ, to be the same thing; but the fellowship which we have with Christ [participation quam habemus cum Christo] is the consequence of faith [fidei effectus]." 42 Union with Christ does not provide a basis for God to discern in us a righteousness imparted; rather, on the basis of justification we are made partakers of Christ's vivifying life. The same act of faith that constantly looks to Christ alone for justification looks to Christ alone for sanctification and glorification.
Thus there are not two sources of the Christian life: one that is forensic and found in Christ alone, and another that is moral and found within us. Forensic justification through faith alone is the fountain of union with Christ in all of its renewing aspects. We are justified through faith, not through union with Christ. ... Imputation and union with Christ are, rather, two inseparable aspects of one and the same divine grace: the one is not possible without the other." 43
Therefore, Calvin speaks of a "double grace" in fellowship with Christ:
Christ was given to us by God's generosity, to be grasped and possessed by us in faith. By partaking of him, we principally receive a double grace: namely, that being reconciled to God through Christ's blameless, we may have in heaven instead of a Judge a gracious Father; and secondly, that sanctified by Christ's spirit we may cultivate blamelessness and purity of life.44
This double grace entails a "twofold acceptance": Our persons are justified apart from works by the imputation of Christ's righteousness received through faith, so that or works can be justified or accepted by the Father not as meritorious but as the fruit of justification and union with His Son.45
 John Calvin, Commentaries on the Epistles of Paul to the Galatians and Ephesians, trans. William Pringle (repr. Grand Rapids: Baker 2003), 262
 [Francois] Wendel, Calvin: Origins and Development of His Religious Thought [trans. Philip Mairet (Durham: The Labyrinth Press, 1987)], 258
 [John] Calvin, Institutes of the Christian Religion, 3.11.1
 Ibid., 3.17.4-5
It can be seen that both authors quote almost the same passage in Calvin's Institutes, through for different purposes. Ryken utilized that quote to mention that union with Christ is the foundation for both justification and sanctification. Conversely, Horton utilizes that same quote to show that both imputation and union are fruits of justification through faith. Ryken states that union precedes justification, while Horton states that justification precedes union which is the other side of the coin of imputation.