Sorry mshilarious.
It would help if we knew each others background a bit more. I am not sure what kind of experience you have from electronic circuit design. As for me I have been working and hobbying in and around electronics since the 70-s.
Not trying to start a flame war, but I have to slightly object to your wordings. Regardless, in a typical Y-cable this should not be a problem unless you happen to end up in special circumstances. I remember visiting an electro steel plant, where the currents are several thousand amps, people with pacemakers were not allowed and it was suggested that we would leave our wrist watches outside as they might be magnetized and stop working. A ground loop there would probably fry just about any kind of equipment.
... It's not really, it's just a extra wire from the same ground point to the mic. Since there is no difference in potential with one lead or two, it wouldn't matter.
Actually, difference in potential is not the point with a ground loop. Magnetic fields induce a current going through the loop. This current can in turn influence other parts of the electronics inside the box.
... Besides, the job of the shield is to collect RF and dump it to ground.
Sort of the opposite. The main purpose of the shield is to stop RF from reaching the signal conductors, not necessarily to dump it to to ground although to a certain degree that is happening as well.
... If the two lead shields had a substantially different ground point in the box, you would have a point, but they shouldn't.
It sort of depends on the box. I´ve looked inside quite a few. Some boxes has different paths for the ground inside the box, some are carefully layed out to not have different paths. And the problem is that a large enough induced current going inside the box might in turn induce a signal to other circuits. Now, to be clear again, this is normally not a problem with a Y-cable at all but if it ever occurs, try breaking the ground loop by cutting the pin1 / shield at one of connectors. Often enough DI-boxes and such used on stage has a breaker for this called "ground lift". It is very common to have ground loop problems on stage and the universal solution is DI-boxes with transformers. The problem is that cheap boxes has cheap transformers that modify the sound in a non-likeable way. Expensive transformers often only sounds good although you do get increased distortion ( as measured by instruments ) towards lower frequencys.
So it depends on the purpose of splitting the signal. If you want to split a microphone signal to many outlets the best solution is an active splitter. This has electronics inside. This might happen at, say, a press conference where dozens or reporters should get a split of the podium mic. In a controlled stage situation the standard solution is splitters with transformers. The sligth degradation of the sound is not noticeable. If you want to really compare two mic preamps side by side I would say that a Y-splitter is the solution as long as you check the output impedance of the microphones. It is also the quick and cheap solution.
// Gunnar