It’s 100% financing—the same strategy that pushed many homeowners into foreclosure during the housing bust. Banks say these loans are safer: They’re almost exclusively being offered to clients with sizable assets, and they often require two forms of collateral—the house and a portion of the client’s investment portfolio in lieu of a traditional cash down payment. In most cases, borrowers end up with one loan and one monthly payment. Depending on the lender and the borrower, roughly 60% to 80% of the loan can be pegged to the home’s value while the remaining 20% to 40% can be secured by...