To say you can not record ambience with only two mics is preposterous.
And no one has said that. What has been said, by me anyway, is that, alone, is not the way good live recordings are made. There are probably exceptions, but one hasn't been presented here as of yet. If you just wanted to record the ambience of the room, I suspect a pair of stereo mics would do the trick. If you want to record the sound of instruments in that ambient space, the way we would hear them in the audience, it just doesn't come out right. I'm not sure what the problem is. I know the human brain has a remarkable ability to focus on some sounds and "turn down" others. I know this is true of listening to music, or the single voice that is speaking to you in a crowded room. I know we also have a remarkable ability to adapt to rooms, and filter out much of their reflected sounds once we become accustomed to them. What I don't know is why these mechanisms work for the room were in, but don't work for recordings. But it sure is the case in my experience. Set a pair of stereo mics up, 10th row center at your concert, and while the noises of the crowd shuffling, breathing and whispering around the mics will be far too loud, the sound of the pianist, bass player and horn will be too distant. There will be a bad balance of direct and reflected sound. And so when recording live, most recordists -- and by most I mean the overwhelming majority -- will capture room ambience, reverb, whatever you want to call it, and they will also capture the sound of the instruments from mics that are much closer to the instruments, then they will mix it together.
Again, go listen to Barry Diamet's samples on his site. This is very close to what you all are talking about. The closest, and the most successful example I've heard; a stereo pair of microphones recording ensemble performances in a performance space. But even then, to get a more natural balance of instruments and voices in the ensemble (and, I assume, a more natural-sounding balance of direct and reflected sound), the mics are
on stage, and the performers are arranged in a line with the quietest sounds closest to the microphone. Barry offers descriptions of his technique, photos, samples to download. Its a great education in what can be done with a stereo pair, but it is still a compromise. If somebody out there has set a pair up in the hall, recorded a live performance and gotten really good instrument sounds and the natural reverb of those sounds in that hall, please, point me to it. I'd love to hear the recording and a detailed description of the recording methodology. The latter shouldn't be hard if the former exists, because the recording will have been unusual and a bit of a breakthrough. Someone will probably have documented it, as Barry has.
And by the way, my temper is fine, in spite of the fact that Mark has attacked my listening experience (again) and he and Myles have both called me "Teflon." It's hard to get angry at guys who are upset because you have answers. You just remind yourself that they're not the only answers and wonder if they're really mad at you, or mad because they have no decent answers of their own.
Tim