While re-reading HOH: S, it occurred to me that these 2 groups basically revere/draw power from the same entities, so, if they adhered to the house Ethos, Muspelli could become members of the order.
Or the tytalus could have goals (and do OR in that direction) similar to the muspellis.
This could quickly degenerate into another Schism War 
Has anyone done anything about this? Especially if you played Dies Irae, how did the Tytali react to the Muspelli's actions?
Just some random musings... 
Our group did have a chance to run the Twilight of the Gods scenario, and while we eventually learned of some Hermetic Magi aiding the Muspelli, none of them, that we know of at least, were members of the Titanoi. Its been some time though so it might not be accurate.
I mean, Twilight of the Gods does suggest a couple of groups from the Order that could be aiding them, most notably the Titanoi and Path of Strife Criamon. Frankly, Wilderist Bjornaer could easily be convinced too IMO. Its rather sad we didn't delve too deep into that dimension of the scenario...Lots of parts of it were left out actually.
Would love play it again some day, in full.
That being said I will be playing a Makhai (Greek Muspelli) in a Theban game soon sooo...Who knows what might happen!
Not quite from the same entities. They do both draw power from Titans, but from two different groups of Titans.
Besides, the Titanoi make use of all sorts of spirits, not just the Titans.
I think it's relevant that the Titanoi approach the relationship from a much better position than the Muspelli. Hermetic magi are basically trading favors (vis for effects) with the titans, while the Muspelli are basically worshippers. The Tytalus would probably disdain Muspelli for their dependence on the Titans, and have the good sense to realize that if the Magic Realm was ascendant it would mean bad things for magi, who currently have a relatively free hand.
Why would magi have any idea of the possible metaphysical changes that an ascendant magic realm mean? This is the stuff of theories, where I probably wouldn't provide an answer to in game, unless a player invested in an incredible lore score and was really creative in his metaphysical explorations to bring about a real answer.
I think it's more a case of going with what you know rather than upending the entire apple cart on speculation that something might be better. Most of the legends of the pre-Titanomachia state of affairs don't make them look particularly good, though it's admittedly biased information (told through a Faerie lens). But even the simple act of loosing the Titans is likely to be apocalyptic.
The Muspelli are doing what they do because they're generally misanthropic, the Tytalus are definitely far more socially oriented, so they'd have a lot more to lose. After all, even if the Titans are loosed, that doesn't guarantee anything other than a prolonged struggle between Magic and Faerie all over again.
Is that really the case though? In Greek Mythology the humanity follows a certain pattern of degradation through the ages: Golden Age, Silver Age, Brazen Age, Iron Age. The Titans (Kronos specifically) presided over the Golden Age and its people. Likewise, Atlantis was by all intents and purposes not a bad place to live in and was ruled over by Atlas before Olympians sunk it during Titanomachia.
Its not exactly a leap in logic for someone to assume Magic ascendant would be better either for humanity or for magic practitioners in general; far as their reasoning for joining or aiding the Muspelli goes.
According to Greek legend (Heisod), the men of the golden, and following silver ages, were destroyed by Zeus because they refused to worship the gods. Then in the Bronze age Zeus created humans from an ash tree.
That means that current humans are NOT THE SAME creatures as the humans of the gold and silver ages. They are not the creation of the titans, but the gods, so why would the titans treat them the same?
The golden age 'humans' lived long lives in youthful bodies and became guardian spirits upon death. It basically seems these were Magic Realm creatures (Magical Humans) that could become Daemons after death. It makes sense that they would flourish when the Magic Realm was ascendant.
This is disregarding the likelihood that Greek legend is entirely wrong (humanity descended from Adam and Eve, created by God) and the humans in Mythic Europe are the same humans that lived before the flood, aside from God limiting humans to 120 years. Maybe the Titans made their own humans as servants or something and Zeus just took credit for human existence by saying he made them from trees, because faeries are like that.
Basically it seems doubtful that the Titanoi would take Greek legend at 100% face value. Titans and gods can lie, legends can be false (or faeries will try to make false legends happen). Tytalus magi especially should expect that the parties involved are trying to manipulate them and analyze situations on a purely cost/benefit basis.
Overturning the entire world order has a huge cost, and the benefit is extremely difficult to calculate. The Muspelli are basically driven by spite, so they don't have any regard for the costs involved.