Suppose I have two fans in my two story house, one in an upstairs window and another in a downstairs window. I want to cool my whole house; it is cooler outside than inside. Is it more efficient for

1. the upstairs fan to blow cold outside air downstairs, with the downstairs fan blowing it back outside,

or

2. the downstairs fan to blow cold outside air upstairs, with the upstairs fan blowing it back outside?