I am no longer of two minds on this matter. This week I read Denny Burk's 2001 DTS thesis , and I have made something of an about-face. Denny Burk contends that there is no anaphoric link between MORFHi QEOU and EINAI ISA QEWi, and while I previously favored his view by a slight margin, I now find it so compelling that I can no longer accept N.T. Wright's grammatical understanding.
Burk's thesis helps one recognize that there’s little to no reason to believe that there is an anaphoric link between the two phrases in the subject text. Not only does Burk point out that there are many infinitives in the NT that are not anaphoric, but he offers the following in relation to the accusative specifically:
“There are many non-anaphoric examples of the articular infinitive in the accusative case as well–indeed, many more than in the nominative case. In fact, it is difficult to construe an anaphoric reference for the majority of the accusative examples of this construction.” (ibid, p. 47)
If Burk is correct here, then the burden to demonstrate an anaphoric link falls on the proponent of that view. This is esp. the case since, as Burk points out, “…most articular infinitives indeed do not denote anaphora…” (ibid, p. 49).
N.T. Wright doesn’t satisfy that burden, as Burk demonstrates in his thesis. It might be possible for proponents of Wright’s view to meet their burden if the article were otherwise seemingly unnecessary, but, as Burk demonstrates, the article was critical for a reason that had nothing to do with anaphora. As he explains:
“…the grammatical context of the sentence requires the presence of the article in this particular infinitive phrase. If the article were not present in Philippians 2:6, the sentence would make little if any grammatical sense…the article is required in this context as a grammatical function marker to distinguish the accusative object from the accusative compliment.” (ibid, p. 50)And
“In such reversed order situations where neither of the accusatives is a proper name or pronoun, the presence of the article is syntactically required in order to indicate which accusative is functioning as the object. Such is the case at Philippians 2:6.” (ibid, p. 52).So, at Philippians 2:6, Paul had to include the article to indicate which accusative is functioning as the object.
Is it possible that the article is doing double duty here, i.e. marking the object and also establishing an anaphoric link between MORFHi QEOU and EINAI ISA QEWi? Perhaps, but those who would insist that this is the case have the burden to prove it, and I have yet to see anyone rise to meet that burden.
Conclusion: Until compelling evidence is offered to suggest otherwise, we have no reason to assume that MORFHi QEOU and EINAI ISA QEWi speak of the same reality. We can therefore embrace a translation of Philippians 2:6 that incorporates the best of Hoover’s argument (the grammatical/syntactical features of the double-accusative idiom), with a meaning of HARPAGMOS that comports with its cognates, e.g.:
Although he existed in God’s form, he did not consider equality with God as something to be seized/grasped for
 THE HARPAGMOS ENIGMA: A PHILOLOGICAL SOLUTION, by Roy W. Hoover, Harvard Theological Review 64 (1971), pp. 95-119
 When I say that I was of "two minds", I mean that I was formerly undecided between the two alternatives that I had come to view as the two most likely renderings of the Greek at Philippians 2:6, namely:
(a) Although he existed in God’s form, he did not consider equality with God as something to exploit
(b) Although he existed in God’s form, he did not consider equality with God as something to be seized/grasped for
I now favor "b" as the best rendering of this controversial verse.
 The Meaning of HARPAGMOS at Philippians 2:6, by Denny Burk, DTS Thesis, 2001