If I recall correctly, the ability of two unmarried taxpayers who own a home together, to deduct interest on a $2 million mortgage ($1 million each) is the result of a couple of tax court cases that ruled that the $1 million limit was per taxpayer, not per loan instrument.
I just reviewed the language of the TCJA and the section that lowers the limit for acquisition debt incurred after 12/15/17 does not also modify the language to specify loan amount rather than personal indebtedness amount. Since the law is silent on this particular "loophole", then I assume it is not changed, and two unmarried taxpayers can deduct interest on up to $1.5M mortgage (750K each). But I am not an attorney.
