I want to be able to make a fan change direction depending on what side of a wall is hotter but dont know much about coding
First, figure out how you're going to work out the temperature thing.
Temp doesn't matter just the temperature difference
You can't have a difference without two readings,
Then find a fan that can run both ways. I've never heard about such fans. Normally the propeller is designed like aeroplane wings, optimized for one direction and the motor only runs one way. Use two counter mounted fans and power either of them, one or the other, never both. The none powered fan will cause some resistance but will idle backwards.
2 fans is fine side by side. I just don't know how to write the program
You're not going to get anywhere doling out requirements of what you want to do like an IV drip.
You don't know how to code; that's cool. But do you have electrical/electronics and mechanical skills? Do you have a list of parts (which Arduino you have? what's powering the fans? are they brushed DC motors? brushless DC? AC? how will it be wired? is there a user interface? how are settings adjusted and stored? ... )