They come at the question differently, but I don’t see how they could both be right.
HIstorians of Christianity widely acknowledge that Marcion compiled the first authoritative collection of distinctly Christian writings… . In doing so, he defined for the first time a biblical canon — that is, in the useful distinction made by Metzger, not just a “collection of authoritative books,” such as a circulating set of Pauline letters, but an “authoritative collection of books,” with set limits that clearly signaled a unique status for the texts included. Marcion clearly intended his First New Testament [sic] to serve as the touchstone of Christian belief and practice at a time when these were still quite fluid and conveyed in a primarily oral environment. Although we cannot be sure that Marcion himself ever referred to this collection as the “New Testament” … it serves as an appropriate designation for Marcion’s two-volume set of authoritative texts… .
BeDuhn, The First New Testament (2013), p. 4.
I am curious whether anyone can cite a text in refutation of Lieu’s negative claim.What these authors [Irenaeus, Tertullian, Epiphanius, author of the Adamantius dialogue] DO NOT DO is to credit Marcion with holding his ‘Gospel’ and ‘Apostolikon’ together in the way that they themselves were doing with their authoritative counterparts, as a single ‘New Testament.’ [footnote: Epiphanius had possession, probably temporarily, of ‘two books’ of Marcion (Pan. 42.10.2).]
This contrasts with modern discussion, which often has concentrated on whether Marcion initiated the idea of a ‘New Testament,’ namely the combination of new authoritative writings of different genres into a single, separate, corpus; that he did so has routinely been seen as the converse of his supposed ‘rejection of the Old Testament.’ However, such models are too precise and introduce fixed concepts that are anachronistic both for Marcion and for his opponents. The initial charges against him were that he denigrated the creator or the one ‘spoken about in the law and the prophets’; the defense by his opponents of the essential harmony between ‘Old Covenant’ and ‘New Covenant,’ and their complaint that Marcion sought to establish a division between these, go hand in hand. Hence, the model and the language are theirs [=the Church’s heresiologists], especially as increasingly these ‘covenants’ come to be conceptualized as documents.
Judith Lieu, Marcion & The Making of a Heretic (2015), p. 186.
There are several layers of this topic to consider. Clearly enough, the Apostolikon is reported to have been a collection of distinct letters. But were these somehow combined with the Evangelion? Combined physically in a codex book, or “two-volume set”? Or rather conceptualized as a single body of scripture? A “canon” or “bible” or “New Testament”? Do we have any evidence at all in support of BeDuhn’s assertions?
If Lieu is right, one ought to at least consider the possibility that Marcion’s “two books” did not begin to circulate at a single moment in time. One might have emerged long after the other.