Servo Mixing
I run mechanical brakes on my E-Buggy. Clutch as well (sometimes) Testing..
Anyway. This is a matter of setting the radio up with the 3rd channel.
Mixing is what I think Futaba calls it.
It works great on my system and the manual really allowed me to fix this
in a matter of minutes. Save the car configuration and you won't loose it.
Also, if your radio allows, you can create a configuration with mixing and one
without.
Check out your manual and it should tell you the details.