At this point we don't know anything for sure. The development and marketing teams of the games tend not to let in on mechanics like EVs and IVs, so they're things we'll figure out ourselves come 17 November.
Though if the process is anything like the Kyurem fusions we saw in B2/W2, then Necrozma will be considered the 'base' form and the fusion will inherit its characteristics. But we do already know that the fusions keep Necrozma's ability, which is different from Kyurem's fusions. Perhaps there will be more variance.
