At first Duncan sees them through a net in the hull of the no-ship, which is standing on Chapterhouse. Because of this some people have got the idea that they are outside space (and time). There are many indications that the are just living on some planet, and only use the net, to see and control things from a distance. From the last chapter we can conclude that they have met Bene Tleilaxu Masters face-to-face, and that these masters saw them as ordinary Face Dancers. Also the garden, in which they are taking care of their roses, gives an impression that they are in a real world, just as the rest. But also the fact that they are Face Dancers, makes it clear that they do not live outside space.
Furthermore, they seem to have certain abilities, which go beyond the abilities of the NFD made by the original BT. Of course, it is possible that the NFD did have the same latent abilities, but that they were never activated, maybe because they did not acquire enough personalia to become independent, or never given enough freedom to become liberated by their bond. Note that NFD that replaced Tuek was staying in that role longer than normal.
Duncan immediately start thinking about how this knowledge can be used for defeating the Honoured Matres, and then comes to a full Mentat Projection, stating: someone "out there" has found another way to use Holtzmann's theories. It seems that the 'someone' is referring to Marty and Daniel, because a little further he says: Who were those people he saw? Strong enough to drive out Honoured Matres. He knew it for a Projection datum. Combining these facts, one could conclude that the Net is created by a novel application of Holtzmann's theories.
It is clear that when the No-ship entered foldspace, Daniel and Marty tried to take over control, and Duncan could only prevent this by clearing the databases. Apparently, the possed the possibility to not only see, but also control No-ships with engage nullfields. The one who controls space travel controls space. Marty says that she had picked a planet, one that would be a real test of their abilities. That means that they could pick from a host of planet, and that it was their plan to bring the No-ship to this planet.
Hi, Gang, Danny:
Now, now, Danny; lots of people have the precise same reaction when they first read Ch:D!! When I first read it, if memory serves, I was so vexed at the ending that I threw the book across the room in a fit of temper (knowing that FH wasn't going to be writing any explanations of the dreaded M&D anytime soon :-)
Anyway, Preacher: Don't let anyone tell you, including myself, that they know who or what Marty and Daniel ("M&D") are, because they do not; the only person who knows that is dead - the author himself. Perhaps his son, Brian Herbert, either knows from conversations with his father; or perhaps it was in his notes, but other than that, there is NO ONE. However, many, many theories abound, and it is often hotly contested on this ng. So, to summarize those most popular (you can also read the FAQ, which lists some more detailed "descriptions" of people's pet theories), see below:
Here's what we DO know: Marty and Daniel are "New" Face Dancers, with the ability to absorb not only the physical characteristics, but the "personae" of the people they copy. This is made clear in the last chapter of Ch:D. Obviously, they have escaped BT Master control, either on their own, or through the destruction of Tleilax. <shrug>. In either event, they have escaped to what appears to be a different dimension or existence, or some other plane of reality - this existence is unexplained. However, it is stated in Ch:D that the Duncan could see him because he was "stretched so thin," which implies that because of his serial-lives incarnation, that this longevity (via serial lives) somehow gave him the ability to "see" Marty and Daniel. It does not, however, explain why this same ability is not present in Scytale, whom we know to be just as old, via the same means. Yet another interesting FH discrepancy!
Insofar as pet theories:
I think this covers the most popular theories. Numbers 3 and 4 are primarily "fueled" by the scene in which Duncan is having a vision, or a dream (or whatever) in which he sees "weapons," and also sees M&D. Duncan contemplates that whoever those "people" are, they are strong enough to "drive out" the HM (although it does not say from where - the Scattering, or their current infestation in the OE). There is no reference throughout either HoD or Ch:D to any NFD created by the Scattering-BT, only those created by the OE BT. In fact, other than the scene I just mentioned, (Duncan and the weapons-dream/vision), there really isn't anything to support any of the theories, or suppositions, or hypotheses, other than FH's clear statement that they were FD, period. NFD whom had, as they themselves state, absorbed a "bunch" of personalities, yes. But there is nothing more upon which to go, in attempting to sort out M&D.
There are also numerous citations which, in my opinion - logically, contradict the theories of 3 and 4, above. For example, the many references which indicate that the HM were running from a biological weapon/agent; and the fact that the OE BT have to learn of the abilities of the HM from their own NFD infiltration, and not from any of the returned BT. It does not, to me, make sense that the Scattering-BT knew of the HM; created biological weapons against them; initially used M&D (somehow) against them <perhaps>, and then, only "ignorant" or "stupid" BT returned to the OE BT without bringing either a word about the HM; or the biological weapon, etc.; and worse, sat back on BT and allowed their world to be incinerated by the HM - especially if they'd been successful in driving the HM out of the Scattering. (Especially as the "returned ones" happened to BE on that world while it was being incinerated).
Anyway....I wish to stress that the last paragraph is my opinion only, and should be taken as such. The previous paragraphs are, to the best of my ability, an attempt to give you a short version of the numerous theories abounding as to who and what M&D were, and an equally short version of what appears to be stated in Ch:D, and what isn't. Okay? I'm quite certain (as the BT appear to be a fascinating topic for some folks on this ng) that others will post comments, as well; IMHO, at the end of the day, you take what FH wrote, think about it and decide for yourself who or what YOU think they are. :-) Because unless BH is going to release his father's notes, or FH is coming back from the Dead, we shall never, never, KNOW.