I use this a lot in my campaigns.
For my latest Waterdeep game, the clergy of Lathander is split down the middle regarding beliefs about his origins:
- Those who believe him to be reincarnated Aumanator, old Netherese god of the sun, keeper of time, and master of law. This faction would like to completely displace the Church of Tyr as the judicial branch of Waterdeep.
- Those who believe him to be an "imported" god of elven origin, alongside Selûne and Sune. These tie that trio to Corellon, Sehanine, and Hanali, who all have matching portfolios. They do not believe in the more rigid aspect of Aumanator-Lathander.
There are high-ranking priests of both sides, so there is a "dawnmaster" and an "anti-dawnmaster", as you described. Lathander responds positively to attempts of
commune from both sides. This is another god is simply coopting the spell and impersonating Lathander in the case of group #1. This idea of coopting came from the
Theros book, which has great advice on all divine stuff.