I'm going to answer my own question on the Medicare wages part. On the IRS website, they have an example in the self-employed page that sheds light on this issue and specifically says "Each individual will calculate Additional Medicare Tax based on his or her own wages without regard to the income tax treatment of wages as community property income." So no need to re-allocate for Additional Medicare Tax.
For NIIT, I'm still not sure, but possibly Form 8960, Line 7 is where such an adjustment should occur (assuming it occurs at all, which maybe it doesn't as the Additional Medicare Tax above indicates is possible).
