If I understand this correctly (big if)...
The 2012 decision you (@Htown3514) referred to was overturned in 2016.
https://www.forbes.com/sites/anthonynitti/2016/08/01/irs-increases-marriage-penalty-unmarried-cohabi...
Note that I have no idea if things have changed *since* 2016, and am still figuring out the actual answer to this question... but I'm leaning toward trusting the OP (@o2dunbar) who said they asked two CPAs who both said two single joint-owners could each claim mortgage interest deductions up to the single-filer limit.
