Using multiple LOCs for investment properties

[L Bauer]

I have a LOC secured by my house (primary residence) and another LOC with a slightly higher interest rate secured by a rental property. Both LOCs were established before Dec 2017. Keeping the new mortgage interest write-off tax changes in mind, if I want to buy another investment property, which LOC should I use? Can the mortgage interest on either one be included as rent expense when I file taxes? As a side note, if I run up the full balance on my house LOC, I will be under the 1M limit, but I will not be using the money to improve the property that secures the mortgage. Any advice is appreciated!