I do like the idea of having a primary domain for spontaneous spells and a secondary domain. It's more interesting than straight up having two domains.

I'm wondering if it might also make sense to take a page out of the wizard book and restrict spelsl somehow. Maybe have them pick another domain or two that they can't cast from? Flavorwise that makes sense, but I don't know domains off the top of my head and I'm sure there are some domains with no good spells.