You could also write in spin off religions. Religions that your one true religion acknowledges, but focused on different aspects. Like in Norse myths there are the Ice giants, which you can give their own followers.
Or you could add panthe with desginatedd responsibilities. For example your Zeus and his family tend to the humans/greeks or other culture of choice, while Zeuses brother and his decendants look out for somone else.
Or you could do the whole false god bit, and say that the beings of power that your group A worchiped were not legitimate, or were unconfirmed in legitimacy. So rather than having one true religion you have one religion with more circumstacial proof.
all of these will let you incorperate multiple religions, with all the strife it brings.