to be qualified residence interest it must be acquisition debt (AD) for the taxpayer's main or second home.
the debt must be secured by each of the homes for which AD was incurred. Refinancing of AD is AD only to the extent it does not exceed the principal outstanding on the loan immediately before the refi. this would seem to mean that any refi done where the balance exceeds the remaining balance on the mortgage on the home refi'd and secured by that home is not AD and the interest wouldn't be deductible. however, it would seem if both homes secure the debt then as long as the amount of the refi doesn't exceed the total of the remaining balances on the separate mortgages all the debt would be AD and the interest would be deductible.
