I have mine set to the boat switch, where BSF is the right controller and BSB is the left controller. Just assign those buttons as modifiers for the WSO (you want them to be regular buttons for the pilot) and you can use the same buttons as the front seat for either selected.
The only catch is that for common functions (F1-F10, eject, the UFC if you have that mapped) need to be mapped to the same keys with the modifiers as well. Otherwise, they won't work when you have one of the modifiers enabled.