1. Jaghut are not native to Omtose Phellack. They usurped it, and were capable of doing so only because Togg was lost (timeline-wise it makes no sense, because in Midnight Tides we see Gothos performing an Omtose Phellack ritual after the sundering of Emurlahn, and Mael says it's "Ice. Jaghut answer to everything", but hey, timeline is not important, right?). At first it was ruled by Togg and Fanderay - the Wolves of Winter, also worshipped by Imass. Additionally, Silverfox and her T'lan Imass are concerned about dangers (renegade T'lan Imass) to the Throne of the Beast, originally belonging to Togg and Fanderay. It seems clear that the Wolves of WInter ruled/originated from (or even created - usually it's Olar Ethil who takes credit for that, but Togg and Fanderay are much older than elder gods) Tellan or one of its aspects. So we have a pair of gods, who are aspected to... two elder warrens?
As for essential quote-fu: end of Chapter 19 MoI, Toc/Togg is talking to the Seer (actually to Mhybe, but he thinks it's Seer):
Quote
you never understood it. Not true winter, not the winter that is not
sorcery, but born of the cooling earth, the dwindling sun, the
shorter days and longer nights. The face I see before me, Seer, it is
winter's face. A wolf's. A god's.'
And few sentences later, this:
Quote
Fangs bared. Not even your dear mother, who holds me so tight,
dares challenge him. As for your Omtose Phellack—he would have
confronted you at that warren's gate long ago. He would have
denied it to you, Jaghut. To all of you. But he was lost. Lost. And
know this, I am helping him. I am helping him to find himself. He's
growing aware, Seer.
2. The aspects of Tellan and Omtose Phellack may at first look like complete opposites - fire and ice. But let's give it a deeper look. If Omtose Phellack is winter, which is said pretty clearly, then that would make Tellan its opposite - summer. But winter-summer is a whole different type of opposition from ice-fire. If there is fire, there can be no ice, and vice versa. If there is winter, summer will come soon. And then winter again. And so on. I am talking here about seasonality, which brings us to what I think is the true aspect of Omtose Phellack and Tellan. Time. With Omtose it was discussed on many occasion, but I think we will all agree that aforementioned scene from the prologue of MT, with Gothos performing ritual after the sundering of Emurlahn, makes it rather clear - the ice of Omtose Phellack has the power to freeze time. It does so by halting the evolution of magic in Lether. But what about Tellan? Well, look at the T'lan Imass. The ritual of Tellan is opposing time itself, by keeping them undead. So, magic of both Omtose Phellack and Tellan is capable of affecting the flow of time.
3. The last thing that I think proves that Omtose Phellack and Tellan are closely connected is worship. The worship of Jaghut by mortal Imass. We learn in MoI that Jaghut tyrants were treated as gods by many of the Imass tribes. And as we all know, in the Malazan world worship is not a small thing. Faith is a power in itself and creates a bond between a human (or, in this case, an Imass) and his god. So maybe that is how Jaghut gained access to the magic of time - through Imass. Maybe they shaped Omtose Phellack themselves or maybe it is just how Tellan manifested in their hands?
This is Chapter 15, MoI again, Silverfox explaining the wars with Jaghut:
Quote
there were intelligent beings in the world who exploited the virtues
of others, their compassion, their love, their faith in kin. Exploited,
and mocked. How many Imass tribes discovered that their gods
were in fact Jaghut Tyrants? Hidden behind friendly masks.
Tyrants, who manipulated them with the weapon of faith.
'The rebellion was inevitable, and it was devastating for the
Imass. Weaker, uncertain even of what it was they sought, or what
freedom would show them should they find it… But we would not
relent. We could not.'
So that's pretty much my theory: There is a warren of time (or seasonality), and Tellan/Omtose Phellack (actually,Phellack doesn't sound that different fromTellan
