Costa had nothing to do with Social Security.  it was cited in a 2002 Tax Court opinion that did deal with this issue, McAdams.  That case didn't need to consider whether separate residences but continuing companionship still allowed for "living apart," because the taxpayer actually stayed under the same roof for more than a month.  Neither IRS nor the Tax Court is going to say that a happily married couple is "living apart" just because they aren't under the same roof.  Especially when, in the case of the condo couple, they are under the same roof -- just different ceilings. 

https://casetext.com/case/mcadams-v-commr-of-internal-revenue-1