1) Yes, they could and would. Among the mortal priesthoods spread across the Realms, there are constant alliances, pacts, and even armed expeditions wherein faiths whose current needs, not just dogmas, run together/share common ground, work together against common foes/obstacles.
@TheEdVerse Hey Ed, question for you! I know of some documented cases where Helmites & Selûnites have worked together, such as the Fellowship of the Purple Staff. Could and would Helmites and Selûnites work together more regularly? Context in the following tweets on why I ask!

Jul 16, 2024 · 8:47 PM UTC

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2) So, yes, Helm and Selûne clergy can and do work together often. Seldom officially "before all the Watching Gods," but always with the approval of high-ranking priests. Just as the clerical forces of evil deities work together. Always neglected in official lore... #realmslore
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3) ...due to the Code of Conduct and concerns about real-world believer backlash, but it's been in the Realms since before D&D. Interfaith feuds, negotiations, misunderstandings, out-and-out wars, and backdoor skirmishes are an essential part of the Realms. #realmslore
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4) But in your campaign only if you want them to be, of course. #realmslore
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Thank you so much, Ed! I appreciate this so much!
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You're very welcome! My pleasure! Keep the Realmslore questions coming (and if you use Discord, many other fellow Realms creatives hang out at Greenwood's Grotto with me, and answer lore questions).
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