In What Way Does Tertullian Understand 'the Old and the New Testaments' to Have Been 'Originally United'?

We have already demonstrated that Cyril of Jerusalem understood Irenaeus to have been the author of the Praescriptionem, another text that now passes in a Latin translation under Tertullian's name.  There is a reference to Marcion here and the concept of 'separation' which is closely paralleled in Adv. Marc:

Si enim Marcion nouum testamentum a uetere separauit, posterior est eo quod separauit quia separare non posset nisi quod unitum fuit. Vnitum ergo antequam separaretur postea separatum posteriorem ostendit separatorem.

For if Marcion separated the New Testament from the Old, he is of later date than that which he separated, since he could only separate what was united. Having been united then before it was separated, the fact that it was afterwards separated shows that the separator was later. [Praescript. 30.9]

In what way does the author think that Marcion 'separated' the old from the new?  The underlying presupposition has to be that the same Holy Spirit now spoke in the evangelists and formerly spoke in the prophets.  To this end, the argument against Marcion seems to be closely aligned with so-called 'kata Phrygianism' - i.e. the new prophesy movement especially popular apparently in central Asia Minor.  

The implication seems to be that the Marcionites were associated with those who denied the Spirit speaking in the new prophets, like Montanus and his prophetesses.  Surely, it was not heretical to suggest that the new covenant was superior to the old.