You want to use two programmable mixes. The first one has the master as Offset and the slave as rudder. Set it to the desired switch and the amount of offset to the position you want the rudder to move to. The second mix has offset as master and the second rudder channel as the slave. Set it to turn on with the same switch as the first mix, and adjust the position and direction as desired. When you turn the mixes on with the switch, they'll move to the offset position you've set.