In the good old days before transmitter mixing, there were mechanical mixers, but the simplest solution was .. .. elevator servo to one side (say left) elevon; and aileron servo to the other side (right). The clever bit is to hold your transmitter at 45 degrees; IE antenae pointing to the side.
With your transmitter pointing to the right - now when you push the stick forward you get full down from your elevator servo and full left out of the aileron servo. As the aileron servo is on the right wing, it now is in full down position - - So both elevons are in full down - hence full down elevator.
And so on - the controls work correctly for all stick positions.
It's a bit difficult to describe but it is very simple to set up and use.
We used to use this system extensively for foam delta slope combat. As we became used to this set up we found that we no longer needed to hold the transmitter at 45 degrees.