Oscar's DPDT switch is probably the easiest, and most effective, solution to this.
It FORCES the other control to go dead.
It also eliminates the need for the Opti-pac.
You can wire both controllers to a single mouse hack, and swap between them manually.
Since you already have the Opti-pac, you could continue to use that; but splice the spinner into the x-axis on the trackball, following Oscar's instructions.