How can one house have 2 mortgages with one owner?

It's like if you needed $5000 to buy a car, and you borrowed $3000 from your father and $2000 from your uncle.

The difference between what your home is worth and the total principle you still owe is called equity. As you continue to make payments, and the value of your house appreciates, your equity grows. That equity can be used as collateral; you can borrow against it.