I do like the idea of one of them setting themselves up as an idol. Maybe thinking it will help steer people from Sauron. The simplicity of being this figurehead is an easy message to understand. A convenient religion. Then becoming a god-king. Then maybe, realising what they’ve become, decries it all snd wanders away, starts a family, raises a child orphaned by some catastrophe done in their name. But you can’t put the genie back in the bottle. Even with their god gone, the cult continues.@Odola
If one of them was of Tulkas people that would be my first guess!
I could totally see them using religion as a weapon against Sauron. If Sauron used a clergy to control the kings under his dominion via a system of belief in which he is the monotheistic god,,it would make sense for them to counter this with rebel cults. Such religions can even exist long after their founders have moved on and take new forms their founding figures never intended. So why shouldn't Sauron do the same and subvert such cults again to his cause?
I like the idea of blue religions which spread among the tribal, more removed folks, those who live afar from the urban centers which Sauronic client-tyrants and Priests of the Eye would inevitably seek to control.
But what religion would they preach? Most likely monotheistic Illuvatar-cults... maybe like early hebrew religion, egyptian Aton-cult, Zoroastrism, Hindu-pantheism with one highest being and many lesser demigods...
Possibly not one Orome-cult and one Mandos-cult... though i could see mortal men turn their teachings back intosuperstition and paganism after some generations...
While the other is so disenfranchised by what their ‘brother’ did, they are this monastic figure leading a temple of Shaolin-esque followers in contemplation, reaching out in mission to serve those harmed by the other wizard. Maybe even training in combat to fight followers of the cult. Imagine how furious snd disgusted he must be with the fallen god figure. Perhaps even thinks he is dead.