The objection to the priority of justification rests partly on the idea that justification and sanctification come simultaneously (though distinctly) through union with Christ – prioritization prohibited. And yet, the problem of prioritizing one benefit before another doesn’t seem to bother the advocates of union when it comes to the rest of the benefits purchased by Christ.
The duplex gratia apparently teaches a double or two-fold benefit that comes through faith in Christ, one being forensic and the other being renovative. And yet, the Standards teach that believers receive not simply justification and sanctification, but also adoption and the other benefits that accompany or flow from justification, adoption, and sanctification.
Q. 36. What are the benefits which in this life do accompany or flow from justification, adoption and sanctification?
A. The benefits which in this life do accompany or flow from justification, adoption and sanctification, are, assurance of God’s love, peace of conscience, joy in the Holy Ghost, increase of grace, and perseverance therein to the end. (Shorter Catechism)
Q. 82. What is the communion in glory which the members of the invisible church have with Christ?
A. The communion in glory which the members of the invisible church have with Christ, is in this life, immediately after death, and at last perfected at the resurrection and day of judgment.
Q. 83. What is the communion in glory with Christ which the members of the invisible church enjoy in this life?
A. The members of the invisible church have communicated to them in this life the firstfruits of glory with Christ, as they are members of him their head, and so in him are interested in that glory which he is fully possessed of; and, as an earnest thereof, enjoy the sense of God’s love, peace of conscience, joy in the Holy Ghost, and hope of glory; as, on the contrary, sense of God’s revenging wrath, horror of conscience, and a fearful expectation of judgment, are to the wicked the beginning of their torments which they shall endure after death.(Larger Catechism)
So, aside from the question of how these other benefits – such as adoption – fit in the duplex scheme, it looks like the advocates of union prioritize just as much as the advocates of justification priority. One group prioritizes justification and sanctification among the benefits. The other prioritizes justification. Rather than being illegitimate, prioritizing is basic to both sides. (It could even be that union advocates prioritize union.)
Old Life 
The duplex gratia has both forensic and renovative aspects. But that doesn’t mean the duplex only pertains to two benefits. The forensic isn’t identified strictly with justification. Neither is the renovative strictly identified with sanctification. Adoption is part of the duplex as well. Justification is entirely forensic, but adoption is not. Romans 8:18-25 teaches that along with its forensic aspect, adoption has a renovative aspect. There is no priority of justification and sanctification over adoption. Adoption manifests union (WLC 69) just as much as sanctification or justification.
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It could even be that union advocates prioritize union.
Not as a benefit of salvation. Categories, categories.
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