Most adults in the main homelands initiate into the deeper mysteries of only one cult, but continue to worship the associated deities and are often lay members of other cults. The cult you initiate to is your patron deity – your personal god. Your Rune points represent the mechanic depth of that connection.
Initiating into a second Rune cult is usually a possibility, but it is rare. This is not lay membership-plus, there is a deep spiritual connection to both deities. One can only initiate into cults that are associated or friendly with the first. And the other cult may impose obligations of devotion and other duties fully as much as the first. Still it is done.
Joining a spirit cult in addition to a Rune cult does not pose these problems and is common. The spirit asks relatively little beyond the initial sacrifice of POW, and regular offering of magic points and sacrifice. I think many campaigns understate how many spirit cults are likely out there.
In my experience running or playing RQG, belonging to more than one Rune cult is fairly rare. I think in the hundreds of player characters I have seen, there’s been less than five player characters initiated into more than one Rune cult. Certainly less than 5%. Spirit cults I see far more common. In a longer campaign almost every player character might be a member of one or more spirit cults.
Note that ancestors are typically spirit cults at best. At best because unless you are doing this through Daka Fal, your tie to the ancestors is rarely going to be as significant as your tie to your personal deity.
So if I am an initiate of Orlanth, I KNOW what happens to me upon death. I join Orlanth in his Palace in the winds, along with my companions and some of my ancestors, until I return to this world. Of course I offer worship to my kin and to the spirits of my kinship community, but my deepest spiritual commitment is to Orlanth.
If I am an initiate of Daka Fal, I am responsible for maintaining that connection and tie with the ancestors. My deepest spiritual commitment is to the ancestors, not the Rune gods.
Spirit cults are easy. They are established all over the place. So there might be a spirit cult to the founder of a community or to an ancient king or to some old hero. That’s not “to the ancestors” – that’s just a local small cult maintained by a shaman or priest. Shamans can easily create new spirit cults based on necessity or convenience.
Your close ancestors are probably not spiritually powerful enough to be able to easily access between worlds. And it is very doubtful they have a lot to directly offer us in exchange for amount of sacrifice it takes to maintain that access.
More likely we just regularly say a thanks to them – and the rest of our ancestors – as part of being a member of a kinship group. Like Lay Membership, but no need for a mechanical treatment.
Now the Daka Fal cultists say that they doesn’t give much, but our collective mortal ancestors give us a tremendous amount. So they maintain a cult of the ancestors, sacrifice POW and magic points and the like, and keep that access open. But for most people being able to gather or dismiss the clouds, bless their crops, and fight their foes lead them towards the major cults.
That’s not really hierarchical or very organized. Its just a recognition that there is only so much to go around.
This is one of the reasons that I think RQ does a much better job with Greg’s thoughts about Gloranthan religion than HW/HQ/13th Age – bandwidth and resources matter much more in RQ than in those other engines. Heck, Greg used to play around with the allocation of resources in one’s unconscious mind just to get a better feel of things.
Are Issaries and Argan Argar an realistic option? The cults are friendly so it is certainly possible. But socially that is not going to be easy in many places.
Orlanth and Humakt? You can initiate to Orlanth and Humakt at once. Both are friendly towards each other. That being said, it is never easy to be an initiate of multiple Rune cults.
Personally I think that the disadvantages of dual Rune cult initiation are rarely worth it, UNLESS I am really trying to game the hero world.