if I have 2 homes, I total the amount of the 2 mortgages?
Home acquisition debt is the amount of the mortgage that was used to buy the home.
Grandfathered debt is a mortgage taken out before October 14, 1987. It is not limited by the mortgage interest calculations.
https://www.irs.gov/publications/p936#en_US_2018_publink1000230017
but ..."on date debt was last secured by home". Doesn't this mean the last refinance, if the mortgage was refinanced?