I think he means to have them on a completely separate network from your wireless LAN.
Either way, link up your 2 NICs with your crossover and give each computer an ip that is different from your other network(s) eg.
ifconfig eth0 172.16.1.x netmask 255.255.0.0
where 'x' is a unique number for each machine. Test the connection by pinging the other machine. If it works, you're good. You can then use whatever method you want to transfer the files. I prefer FTP, since security is not an issue in this case (if you are paranoid, disconnect from the wireless network first) and it is usually the quickest to set up.
eg. (assuming ubuntu/debian)
user@laptop$ sudo apt-get install proftpd
From there you can FTP to the laptop from the desktop machine, using your user credentials and retrieve the files.
After you're done, simply remove proftpd from the laptop.
user@laptop$ sudo apt-get remove proftpd && sudo apt-get autoremove