Heloc Interest Deduction

maybe and the reason is contain in IR 2018-32 example 2

 In January 2018, a taxpayer takes out a $500,000 mortgage to purchase a main home.  The loan is secured by the main home. In February 2018, the taxpayer takes out a $250,000 loan to purchase a vacation home. The loan is secured by the vacation home.  Because the total amount of both mortgages does not exceed $750,000, all of the interest paid on both mortgages is deductible. However, if the taxpayer took out a $250,000 home equity loan on the main home to purchase the vacation home, then the interest on the home equity loan would not be deductible.

what the last sentence is saying is that if mortgages and heloc on the first home exceed the acquisition cost of the first home, the interest on the heloc is not deductible.  The fact that the heloc is on the first home while the improvement is on the second home is what causes the issue 

further then secured debt on first and second home is limited to $750,000 ($375,000 if MFS)