Just as there are many types of weddings, a mixed-faith service can vary ceremony by ceremony. Not just because the couples are different. Or because the religions highlighted might be different.
As with any celebrant-led ceremony, you can tailor the components to fit in with what you believe and want to include.
The ceremony can be 100% religious (or almost). I have conducted a couple of mixed-faith ceremonies, which were, to all intents and purposes, pretty much traditional Jewish wedding services (albeit with less Hebrew). In both cases, there was a Christian reading and/or prayer included. That’s how the couples wanted to play it out, and it worked.
In most cases, the couples want religious elements, despite not being active worshippers. In one case, it was simply to placate parents. Usually, it is because the couple, though not active participants, feel attached to their religion and want to include this as part of their important day.
There’s nothing to stop you mixing religious and pagan elements into one ceremony.
In all cases, the couples need to agree beforehand what they actually want. If they don’t, it’s a recipe for dis-harmony and potentially resentment. Of course, discussing things with the celebrant can clarify how things are going to appear on the day and make it easier to choose.
It may be that the couples want to clear things with their parents too. That’s fine, although you might be walking a tightrope here. Naturally, you want to please everybody (especially if your parents are helping to bankroll the event), but it is ultimately your day. You don’t want to get railroaded into doing what doesn’t sit comfortably with you.
One thing to bear in mind is how “heavy” you want the ceremony to be. For example, when I do a Jewish (or part-Jewish) wedding, I weigh up how much Hebrew should go in. You will need to decide on how many, and which, prayers you include. What sort of rituals will you put in? How about a few spiritual readings? Do you want a balance of religions, or is it OK, if things are a bit lop-sided?
Again, discussing this with your celebrant should illuminate things better for you. You could do worse than to have a word with me!