The dual source option family (Option 31 and Option 43) has been discussed in this chapter. Both are centered on the availability of two sources that enable simpler mixer and IMD measurements, true mode stimulus balanced-differential measurements, and a variety of other possibilities. The hardware parts of the options have been configured for maximum connection flexibility and the very open multiple source control system allows for maximum measurement setup flexibility. The TMS calibration approach is as transparent as possible so switching between dual-drive and single-drive measurements is easier.