OK, here's what to do:
Open an existing (controller-)map or create a new one via the Control Manager's "Map Wizard" which walks you through this process step by step. Add your CH USB-controllers to the map and enable scripting by adding the "CMS Controls".
Push your rudder and the according axis' dialog will be displayed. Uncheck the "DX Mode"-checkbox and leave the "Programmed Function" blank. Repeat this for both toebrakes. Then go to the "CMS Controls"-tab and select the virtual axis "A1": be sure its "DX Mode"-checkbox is marked and select "Device 1" and "R Axis" from the drop-down lists; this prepares this virtual axis to generate the rudder-axis' output (controlled via the script below).
Last but not least open the "CMS Editor" and copy and paste the following script into it (and hope I didn't screw it up

: I'm at friends right now so I don't have any CM at hand to validate my code and ensure there are no syntax errors):
SCRIPT // beginning of script
IF ( [ JS3.A1 >= JS3.A2 ] ) THEN // if left toebrake pushed then
CMS.A1 = 128 - ( JS3.A1 / 2 ); // subtract half of its value from rudder's center value
ELSE // else
// IF ( [ JS3.A1 < JS3.A2 ] ) THEN // if right toebrake pushed then
CMS.A1 = 128 + ( JS3.A2 / 2 ); // add half of its value to rudder's center value
// ENDIF
ENDIF
ENDSCRIPT // end of script
(The lines starting with "//" are not needed but just left in for illustration.) What does it do?
Well, it sets up the left toebrake to generate the left rudder's part of the rudder-axis' output, and the right toebrake to generate the right rudder's part of the rudder-axis' output; if both toebrakes are pushed at the same time, then the lower one scores.
An analog axis has a range from 0 to 255, with its center-position being 128. For the rudder-axis, a value in the range from 0 to 127 means "rudder left", a value from 129 to 255 means "rudder right". For the toebrakes, a value of 0 means "released", a value of 255 means "fully pushed", and there is no center position. To mimic the left rudder, half of the left toebrake's value is to be subtracted from the rudder's center-value, and to mimic the right rudder, half of the right toebrake's value is to be added to the rudder's center-value. It's simple arithmetics - that's why it's looking so confusing

(and I'll compile and test the code once I'll be home again to see whether I've been confused, too

).