Thanks to all of you who responded.

In California, we have a 1% "Mental Health Services Tax" (MHST)  on taxable income over $1 million *per return."  So if a joint taxpayer has a high income year (like a house sale, or lots of stock sales), it pays to split the return into MFS, save up to $10,000 in MHST, and pay the slight cost of a Federal MFS over MFJ.  But that leads to all sorts on interesting complexities, like the one I asked about.

Again, many thanks.