[morzh]

Murf

Shield works as a total drain of all the charges if grounded: no matter where the electric field comes from, the potential is always zero and so no influence is imparted to the wire within. (this is of course onl'y partly true, as the electrical field if changing begets the magnetic field and the wire shield is not much of a protection there, but it all depends on the frequencies).

If however the shield is not grounded, it is fairly useless as the potential is now floating and capacitive coupling affects the wire within (Hum galore!).

Now, the second grounded wire you suggested: well, depends. One thing is when you have a 2-wire signal, of which one is GND. There the coax and a diff. pair a both effective. Better yet a twisted 2-wire pair.
But if there is just one wire that carries the signal and the shield is just the shield, then the second wire simply won't work as it onlhy protects one side of the signal wire, and you need to protect all the way around.
It actually does work if you deal with a PCB (a printed circuit) as the wires are essentially two-dimensional there, and so-called guard traces are indeed used (not that they are always effective, no, but sometimes, and if routed correctly (stitched to a plane well etc) - they may help).