I bought and sold a home in 2018. Do I treat mortgage interest deduction separately? 1st home, interest up to 1million and 2nd home, interest up to 750K?

[DanielV01]

Yes.  Since the first home was purchased (I would imagine) before December 23, 2017, it has the grandfathered-in provision of a $1,000,000 limit for determining deductible mortgage interest.  The new home purchased after that date falls under the new $750,000 limitation.  If your new home mortgage was more than $750,000 of acquisition debt, the deductible portion of your interest will be phased-down in proportion to how much over the $750,000 limitation your loan is.  (You will be able to claim a percentage of the mortgage interest but not all of it).