That does sound odd, since it's possible to detect the state of the triggers independently. The configuration utility for the Xbox Controller driver that I use allows me to map any of the physical axes to the virtual axes X, Y, Z, Rx, Ry, Rz, and Slider, so I guess that's roughly what's supported by the Windows input APIs. I don't really know how it's reported by the controller, though, other than the fact that it isn't standard USB HID.