Hello Confused_in_CA,
my interpretation of the facts are that you should be able to include all the interest you paid on both your mortgages for 2018 (assuming you are filing jointly).
Per IRS Pub. 936 - you take an average your debts, for you this would be $600k ($175k of grandfathered debt & $425k of home acquisition debt since law change). Since your average of $600k is less than $750k you are not limited by limits on home mortgage interest deduction.
Look at and fill in the table on page 11 of Pub 936 for 2017 (except on line 3 where it says $1,000,000 cross off this and change it to $750,000. https://www.irs.gov/pub/irs-pdf/p936.pdf#page=11
This is my opinion and you should conduct your own research.
I hope this helps and thanks,
Jack
