I have my Xbox 360 controller connected using the xboxdrv userspace drivers. One of the features from this that I use is 'trigger-as-zaxis', which makes one of the triggers -z axis and the other trigger +z axis. This is essential for getting some emulators to work.

When configuring the controller in Steam, pressing one of the triggers will bind the entire axis, rather than just the + or - of that axis. When I bind the other trigger, it removes the previously set one (since both are trying to be set to the zaxis, rather than +z or -z)