Look at the Philosophumena (5.16) and its mention of a 'Sethian' interest in the term συγκεκραμένα:
These, and other assertions similar to these, are made (by the Sethians) in their interminable commentaries. They, however, persuade their disciples to become conversant with the theory respecting composition (κράσεως) and mixture (μίξεως). But this theory has formed a subject of meditation to many, but (among others) also to Andronicus the Peripatetic. The Sethians, then, affirm that the theory concerning composition (κράσεως) and mixture (μίξεως) is constituted according to the following method: The luminous ray from above is intermingled (καταμεμῖχθαι), and the very diminutive spark is delicately blended in the dark waters beneath; and (both of these) become united, and are formed into one compound mass, just as a single savour (results) from the mixture of many incense-offerings in the fire, and (just as) an adept, by having a test in an acute sense of smell, ought to be able from the single odour of the incense to distinguish accurately each (ingredient) of the incense-offerings that have been mingled in the fire,--whether, for example, storax, and myrrh, and frankincense, or whatever other (ingredient) may be mixed (in the incense). They, however, employ also other examples, saying both that brass is mixed with gold, and that some art has been discovered which separates the brass from the gold. And, in like manner, if tin or brass, or any substance homogeneous with it, be discovered mixed with silver, these likewise, by some art superior to that of mixing, are distinguished. But already some one also distinguishes water mingled with wine. So, say they, though all things are commingled (συγκεκραμένα), they are capable of being separated. Nay, but, he says, derive the same lesson from the case of animals. For when the animal is dead, each of its parts is separated; and when dissolution takes place, the animal in this way vanishes. This is, he says, what has been spoken: "I came not to send peace on the earth, but a sword," --that is, the division and separation of the things that have been commingled (συγκεκραμένα). For each of the things that have been commingled is separated and divided when it reaches its proper place. For as there is one place of mixture for all animals, so also has there been established one (locality) of separation. And, he says, no one is aware of this (place), save we alone that have been born again, spiritual, not carnal, whose citizenship is in heaven above.My guess again is that thing which stands underneath all the polemic of the Letter to Theodore is the Carpocratian rejection of the canonical gospel of Mark for being a 'mixture' of things related to the Law and things related to the Gospel. They wanted to 'separate' the gospel from commingling. Everything else about the letter follows from this.
In this manner insinuating themselves, they corrupt their pupils, partly by misusing the words spoken (by themselves), while they wickedly pervert, to serve any purpose they wish, what has been admirably said (in Scripture); and partly by concealing their nefarious conduct, by means of whatever comparisons they please. All these things, then, he says, that have been commingled (συγκεκραμένα), possess, as has been declared, their own particular place, and hurry towards their own peculiar (substances), as iron towards the magnet, and the chaff to the vicinity of amber, and the gold to the spur of the sea falcon. In like manner, the ray of light which has been commingled (συγκεκραμένα) with the water, having obtained from discipline and instruction its own proper locality, hastens towards the Logos that comes from above in servile form; and along with the Logos exists as a logos in that place where the Logos is still: (the light, I say, hastens to the Logos with greater speed) than the iron towards the magnet.
And that these things, he says, are so, and that all things that have been commingled (συγκεκραμένα) are separated in their proper places, learn. There is among the Persians in a city Ampa, near the river Tills, a well; and near the well, at the top, has been constructed a certain reservoir, supplied with three outlets; and when one pumps from this well, and draws off some of its contents in a vessel, what is thus pumped out of the well, whatever it is at all, he pours into the reservoir hard by. And when what is thus infused reaches the outlets, and when what is taken up (out of each outlet) in a single vessel is examined, a separation is observed to have taken place. And in the first of the outlets is exhibited a concretion of salt, and in the second of asphalt, and in the third of oil; and the oil is black, just as, he says, Herodotus also narrates, and it yields a heavy smell, and the Persians call this "rhadinace." The similitude of the well is, say the Sethians, more sufficient for the demonstration of their proposition than all the statements that have been previously made.