Connect computers via ethernet rather than wireless This is my current set-up:

Wireless network, multiple macs connected, two of them are standing next to each other. As I want fast file transfer between those, I've got an ethernet cable between them. With IPv4 turned off (on ethernet) on both of them, and Ethernet on one in the service order, this all works. Kind of. 

Because the Mac will decide whether it wants to use wireless or wired, it seems like it will only use the ethernet when it transfers bigger files. Is there a way to always let them use wired instead of wireless?
When there are 2 routes available, I usually just temporarily Turn Airport Off in Network Preferences when I want to force a transfer (or other network connection) to start over the wired ethernet port.