What's supposed to happen is that the baby comes out as either one or the other. Mind you, some aspects of an occult are not tied to the occult trait so the child could be one occult type with features that are normally associated with the other.
There are still some game glitches that can occur with these sort of pairings as well, though several of them have actually been fixed by Maxis, thankfully.
There's also a mod that addresses and tries to fix some of them and also restores the ability to have actual hybrid babies. It is still being actively maintained: https://modthesims.info/d/647863/all-in-one-occult-hybrid-stabilizer.html The mod is likely to not be compatible with any mods that enhance or add to occults, however.