When you want to compare the sound of two things, it's certainly a good idea to shorten the time interval between them as much as you can. And this type of switch could be OK for dynamic (or self-powered) microphones if the switching transients aren't too severe.
But with condenser microphones that are powered through their cables (e.g. phantom powering), there would be an interval after switching in which the capsule in the newly selected microphone is still charging up and the amplifier circuitry is still stabilizing. It might be only a second, but there wouldn't be a clean, immediate transition from one microphone's signal to the other's, if that's what anyone was imagining. Also, with a "make before break" switch as specified, there would be a moment during which current is drawn by both microphones simultaneously, which could disrupt or even (probably not often, but I've seen it) damage the power supply / preamp / recorder.
For all listening comparisons, we must never forget the old hi-fi salesman's trick: If you make one pair of otherwise comparable-quality speakers (or anything else) just half a dB louder than the other, the customer will tend to prefer the one that's louder, though they won't perceive a difference in loudness as such. Instead they will "hear details that the other one doesn't reveal" or "just feel like the sound is more real and convincing" or "a veil was lifted from the sound" or the like. Knowing the trick is no protection; you actually need to check and match your listening levels or the comparison is bound to be misleading. No amount of book learning or golden-earness can help; flip the listening levels, and (if the sound quality is generally comparable otherwise), your preference will tend to flip as well.