It's not easy to use a simple Y-cable because of the phantom powering. You would need a mic splitter, but the standard splitters have a "direct" path that passes the phantom power from one pre-amp and then a transformer-coupled path that passes a version of the signal to the second pre-amp (isolating the phantom power). The two outputs are therefore slightly different, maybe by as much as the difference between two similar pre-amps.

There are more elaborate splitters that have transformers in both paths, and one of these would give you a more accurate (but not identical) copy of the mic signal on the two outputs. Alternatively, you may be able to use an external phantom power supply and then a simple balanced Y-cable going to both pre-amps with their phantom power turned off. This would at least guarantee an indentical signal at each pre-amp input.

Don't forget that a mic feeding two loads is not necessarily going to sound exactly the same as the mic feeding the load of one pre-amp at a time.

Thanks Boswell, I figured it couldn't be that simple.

So really, the truthful way to compare any mic and preamp is one on one using the same sound source but going into their own dedicated paths and setting it each one up as close as possible to what we can humanly hear?

If you wanted to compare different microphones, you could certainly use a dual-channel pre-amp and plug two mics into the two different channels. That way the main difference would be due to the microphones and not the pre-amp channel paths, and, in addition, you could make individual load impedance adjustments for the two mics if they needed it. Of course the two mics could not be positioned at exactly the same point in acoustic space, so, no matter how identical the mics and pre-amps were, there would still be a difference in the output.

This is a different problem from the one we discussed earlier of comparing two pre-amps using a single microphone.