I did manage to figure it out today. Apparently you don't need to use the Combine parameters at all. Here are the basic steps:
1. Identify the physical axis that you want to use as the input. In this case it's My T-Rudder pedals on AxID 7 (Rot Y)
2. Find two axes that are unused. In this case it's AxID 6 (Rot Z) and AxID 8 (Slider)
3. Set the AxID of the two unused axes to the same AxID as the physical input axis (in this case, AxID 7)
4. Make sure that the physical axis has the "Cn" (Axis With Center) box checked
5. Check the "R" (Physical Data Inversion) checkbox on one of the virtual axes
That's it. In the bottom "Physical Axes" section, I set the 'Input' value to "Virtual" for both of the virtual axes. I'm not sure if this is necessary at all.
Voila, that's all that's required. One of the virtual axes will be triggered when the physical axis moves to the left. The other will be triggered when the physical axis moves to the right. For me, this is ideal. I can bind the rudders and left/right wheel brakes to the same pedals axis without having to worry holding a button down. For the purposes of maneuvering on the ground, this suits me just fine.
Here's how my setup looks:
And here are 2 images showing that each of the virtual axes are bound to the left and right pedals respectively