I wasn't able to find any good examples of this method of mixing elevators and ailerons without using hinges or other hacks, so here's my method:

1. Read the latest docs.
   
   
   
   • "Funky Trees" functions and input variables doc
   
   
   • "Funky Trees" 1.9.202 Addendum
   
   
   • "Funky Trees" 1.0 wiki page
   
   
2. Install/activate the Overload mod (Windows, Mac, Android) or find another way to edit your XML files (iOS).
3. Build a plane which will need elevons; usually this will be a delta or flying wing configuration.
4. Add one control surface to each wing. You will probably want this to be close to full span for a delta wing, but for heavily swept flying wings the elevons should be about 1/4 to 1/2 span, at the tip furthest back from the Center of Gravity of the plane (Cg).
5. Edit the XML for each wing:
   
   
   
   • Right wing: ControlSurface: "inputID" = "((Roll - Pitch) / 2)"
   
   
   • Left wing: ControlSurface: "inputID" = "((Roll + Pitch) / 2)"
   
   
6. Fly!

Notes:
- The operator is reversed between left and right wings because SimplePlanes is reversing one of the inputs behind the scenes for one of the wings.
- This mixing function is pretty simple: add the values of the Roll and Pitch inputs (both are [1, 0, -1] inclusive) and then halve the total so that the absolute value of the effective input never exceeds 1. This means that the effective Roll and Pitch inputs are each 1/2 of normal, but since you're using larger control surfaces this shouldn't be much of an issue.

I also tried a normalized mixing function:

inputId = ((Roll - Pitch) / (max(1, abs(Roll) + abs(Pitch))))

which gives full control authority when using only Roll or only Pitch, but prevents exceeding 1 when e.g. Roll is 0.75 and Pitch is 0.75. I found this to be too sensitive but it could be useful for e.g. dogfighters.

Example craft using my elevon mixing functions can be found here:
Dobson Convertiplane Prototype 7