Electrical – How close to a stepper motor do I need to shield the wires

Interference is unlikely to disrupt a signal powerful enough to drive a stepper, but the resistance of normal ribbon cable is a likely problem. On the other hand, interference from the motor cable could disrupt other circuitry nearby, so some shielding might be called for.

Two experiments:
(1) run both steppers off short cables; they will probably work fine. (If not, give the "far" one its own driver amplifier)

(2) Run thick wire (cross section at least 1mm^2) to the far stepper motor to reduce the resistance in series with the motor. There's not a lot you can do about the extra inductance of the longer cable, but at low speed it's not likely to be such an important factor. This should give reliable operation up to a certain speed; but you may have to reduce the rate on this axis a little compared with the other axes.

Yes, motors generate a ton of electrical noise.

In grounding the shield cable, the main thing you want is for any charge that's built up to have some place to go very easily. This means you want low impedance and a big capacity to sink charge. Generally we get this with a big fat wire connected to ground which, in many cases, literally means terra firma. In your home you likely have somewhere an 8' metal rod shoved into the dirt and a 12-14awg wire connecting it to your electrical system.

For your application, you have a floating system. Your bike is more or less insulated from the surrounding environment. The frame is a decent sized capacitor, meaning it can sink some charge without much change in voltage. From your description we don't know anything about the motor but there's a decent chance it's ground is connected to the frame anyway. This would be easy to check with a multimeter. Either way will probably work.

If the shielding is insufficient, I suggest moving the power and signal wires away from each other as far as possible. This is standard practice. If you get really desperate, you might add a choke (http://en.wikipedia.org/wiki/Choke_(electronics)) to the motor drive lines to clean out some of the higher frequency signal.