Once you enter both loans, TurboTax asks some follow-up questions if you click Done in the Home loan deduction summary screen. It will ask Do either of these apply to your loan? Since you originally acquired the home prior to December 15, 2017 and did not take cash out, you would answer Yes. This will apply the $1 million limit to your mortgage interest, rather than the $750,000 that is applied to homes purchased after that date.
I imagine next year there will be a similar question, but it is strangely placed in that you have to go through all the screens in the mortgage section before getting to it.
