I have a soft spot for the topic of people who are dual faith. It’s weird, you know. If you’re an atheist, you get a thumbs up from me. If you’re religious with one faith, you get a raised eyebrow from me. And if you are dual faith, you get two thumbs up from me. It just feels like you’re more open-minded if you are more than one faith.
In the Abrahamic religions (Judaism, Christianity, Islam), having multiple faiths is explicitly forbidden in the Ten Commandments: https://en.m.wikisource.org/wiki/Bible_(King_James)/Exodus#20:1
And you can see this play out historically: when Christians met peoples of other faiths, they didn’t see it as an opportunity for learning and cultural exchange, they saw them as heathens that needed to be proselytized… or eradicated.
Conversely, I feel like Buddhists are the most likely to have other faiths, because Buddhism changed and adapted as it spread throughout Asia, and incorporated elements of the local religions.
Didn’t Islam fuse with Mongolian mythology at one point once Genghis Khan conquered the Turks?