Install FTP server and Client software on each computer.
You use the server software to set up specific folder/directories that you want to share and the other PC will use the client software to connect to you and download/upload whatever. Vice versa if you want to share each others files. You can set you passwords and permissions etc to prevent people from accessing your hard drive, set up usernames and passwords to give to whoever you want to access your PC.
FTP server and client software is available free.