You can't use house #1 as the collateral for both the mortgage on house #1 and house #2, because the bank is smart enough to know that you don't actually own house #1. If house #1 has appreciated significantly from the purchase price, or you have paid off a good chunk of the mortgage, you may have enough equity to take a loan (eg home equity line of credit / HELOC) on that equity to get down-payment money for a second house. That's generally a slow process, unless you happen to own property in a market bubble.