If you start from the position that these are “two divinities” you are always going to run into problems. So start with this proposition – “two names”.
At the Dawn, the Orlanthi worshiped the Sun using the rites that had been preserved through the Grey Age – the rights of the last Light in the Darkness, who was also known as Lightfore in the Grey Age. The Sun was a solar disc carried by the god “Elmal” and the subject of these rites. They may have been taught many of these rites by their High King Elf allies on the Unity Council.
But when the Orlanthi encountered the Sons of the Sun in Peloria, they discovered to their shock that the Pelorians knew secrets of the Sun that they did not. The Pelorians could call down Fire from the Sun, something the Orlanthi could not do. The Pelorians proved with their magic that they could interact directly with the Sun – who was the returned Yelm, whom Orlanth killed and then returned with the Lightbringers.
Many Orlanthi found a way to work with this, and changed their rites. They still worshiped the Last Light and Lightfore, but worshiped him as the Little Sun – Yelmalio. However, a few groups in the Shadowlands continued to worship the Last Light and Lightfore as the conveyer of the Sun, as “Elmal”.
The God Learners viewed this all as Yelmalio, and had the EWF survived that probably would have been the case by the late Second Age. But the EWF was destroyed, and the Shadowlands cut off from Peloria with the Dragonkill War. And so the little Elmal cult rites continued for a few more centuries. It had little magic, little social importance, and was little more than the base Solar acknowledgement the Orlanth cult itself was going to make.
When the Orlanthi that resettled Dragon Pass encountered the Grazelanders and the Lunar Empire, they once again saw the Yelm cult in its full majesty. Not some atrophied empty “Elmal” but the Imperial Sun with all its vast myths, theological richness, and deep secrets.
This shattered the Elmal cult. The Lunars exploited this, encouraging them to worship Yelm directly. The Elmal cult fought with the Orlanth cult, and even betrayed the Prince of Sartar. It looked like civil war would destroy both the Elmal cultists and the Kingdom of Sartar (which was the Lunar plan).
Instead Monrogh traveled to the Hill of Gold and discovered what the First Age Orlanthi had – that Elmal is another name for the Little Sun, aka Yelmalio. Yelmalio is a separate entity from YELM, not from Elmal. He has his own rich myth, his own rich story – which builds upon what the Elmal cult already knew and experienced. Within a generation, Elmal became Yelmalio. Both names are used, although the cultists generally prefer Yelmalio.
We have many names for one deity with plenty of cults – Lodril, Ernalda, Grain Goddesses, even Orlanth and Humakt. But for GAME PURPOSES that is a level of totally unnecessary navel gazing. And to be honest, it only seems to show up with Elmal and the Carmanian Humakt (who is just plain Humakt).
But I don’t think we even have this with Elmal. Elmal and Yelmalio are both Theyalan names for the same entity. What we call the Elmal cult was a pretty superficial contact with that entity, what we call the Yelmalio cult was a deeper contact with the entity (removed from the constraints of being only what could be associated with Orlanth).
And Elmal is pretty academic now. The Sartarites accept that Monrogh’s Vision was true. We are all Yelmalions now.