Basically, there are two actuators that the ECU grounds to run both turbos. You intercept these wires at the ECU and cut them, leaving enough slack on each side to solder on extension wires.
Lengthen the two wires that are on the harness side, and connect them to the middle two pins of your switch. Lengthen the two wires coming from the ECU, and connect them to the right two pins of your switch. (ensure that the lengthened wires from the ECU are on the same pole as the same color wire from the harness)
Ground the left two pins on your switch. Now when you switch it so the pins are always grounded, you will be in TTC. When you switch it so the ECU has control of when the pins are grounded, you will be in sequential.