I think I may have figured it out. If AP "A" is in CMD, then the both Capts and F/O's flight director commands are coming from FCC-A (master).
Under normal conditions, the Left Symbol Gen is displaying info from the master FCC-A (Both FCCs send the same output data to each SG) and the Rt Symbol gen is also receiving data from FCC-A.
That way in a sense, you have 2 sources of the same info to compare.
Well let's say you lose the No.1 Symbol Generator and switch to BOTH ON 2, now you have info from FCC-A displayed on both sides.
Not as much redundancy.
That's all I can come up with.
But there is no question that the A/P can be engaged with a faield onside SG.
Any more....