Part 29: InitiationIt's been a while since I looked into the subject, but from my understanding, in-universe dual-worshiping is rare, but it happens.
See, a part of a child's initiation into adulthood is a kind of spirit quest where they go and discover which god is their patron god. Normally it's perfectly obvious to everyone already and goes off without a hitch. Sometimes though you'll get a kid who qualifies for more than one god. He might be, for example, a strong fighter who gets along well with horses (Elmal), while also being the sort of person who loves cats and can command their respect (Yinkin).
Usually when this happens, he goes off on their little journey of self discovery and find out that, in fact, he is just an Elmali with a penchant for cats. His visions are of Elmal, and while Yinkin might appear in a supporting role, The Cat God never whispers strange secrets in his ears.
Sometimes though the individual has the right runes and both Gods claim him. He ventures deep into the god realm and is taught the mysteries of both The Horse and The Cat. This is, of course, rare, but it does happen. Orlanth (for men) and Eurnalda (for women) are the most likely god to have as a second, for fairly obvious reasons. Minor cults and gods are also common seconds, as their domains tend to be more specific and their runes more likely to be shared with other gods.
Followers of two gods are, after their initiation into adulthood, rushed off to be initiated into both gods' cults simultaneously. They are full blown members of both and are expected to follow all of the cult restrictions. You will never see a follower of both Chalana who heals and Hamukt who harms because it is impossible to abide by the pacifistic oaths of The Healer while following the grim and solemn duties of Death. If the two cults have conflicting practices or obligations, the initiate will have to find a way to oblige both or else lose their membership in one or both. They are not afforded any extra mercy just because they're favored by some other god as well. Though of course, some cults might be lenient for political reasons.
This added responsibility is not without reward, however, as they also gain the magic and secrets of both cults, granting them a range and versatility of magic and knowledge that few others can match. You can imagine how powerful an Orlanthi/Elmali can be as a war leader.
They do start to run into problems later in life, however, where advancement within their cults starts to require more and more dedication. Remember that god worshipers become more and more like their god the more powerful they get. This means that dual-worshipers are torn on a very philosophical level between two fundamental aspects of themselves. Most make a choice to devote themselves more to one god than the other, and so the individual in our example might go on to become a high ranking member in the cult of Elmal while still only being an initiate in the cult of Yinkin. Very few unlock the full potential of both gods and it is very rare to see a full blown dual-godded priest.
Oh, and fun fact: I've seen at least one guy who was a follower of Elmal, Orlanth and Odayla, so two isn't even the maximum.
It's my understanding the reasoning for all this, on a meta-level, is that some players wanted to multi-class their god worshiping, and so this afforded them a way. Players can join as many cults as they like, provided they qualify and are able to fulfill all their obligations.