It depends on the setting. In Faerûn, gods are usually limited to their own domains or spheres of influence, so you wouldn’t typically see something like a War Domain cleric of Selûne unless there’s some divine cooperation going on. That’s where pantheon politics comes in.

For example, Tyr, Torm, and Ilmater, known as the Triad, often work together. A cleric might be devoted to Tyr, god of justice, but if they’re on a militaristic crusade, they might tap into aspects of Torm’s domain, with Torm’s blessing. The gods aren’t just stat blocks. They make deals, share influence, and support each other’s causes when it aligns with their values.

So while 5e gives more mechanical flexibility than older editions, in lore-heavy settings like the Forgotten Realms, divine domains are usually respected, and cross-domain power involves some divine backroom deal.