JP, you can use a trick in Users\Doe\Settings.ini for that.
Assign brakes to one pedal, then open up said file (Make sure you pick the correct subfolder under 'users', ie. the one belonging to your default pil2ot)
Look for:
AXE_RZ JoystickDevice2=rudder
AXE_X JoystickDevice2=brakes
Where the Device2 can differ, ie. could be Device0 or Device1 or Device3
then add a line:
AXE_Y JoystickDevice2=brakes
where you use the same Devicex designation as in the original.
You also may need to swap X and Y values, depending on what your current settings are...
Please note that pressing left or right brake pedal will still just activate brakes, the differential part still only comes from moving the rudder pedals!
Also, after editing, either set the 'Read Only' attribute on the Settings.ini file, or never ever enter and leave the in game Controls screen no more, cause doing so will have that extra line you added removed...
Alternatively, in CH software, you should be able to combine both pedals to one axis, I think... I hope... Maybe...