As to hearing the other plane, I would point out that the rush of air over the fuselage at 500 mph is so noisy that I cannot hear if a family member is behind me talking with me. If you've been in a plane during a thunderstorm I've never heard thunder, even if the strike is close and in front of us. Finally, if you are two or three to take off and the plane ahead of you is a few hundred feet away and is at full power for take off and blasting it's exhaust towards to as it begins it's roll, and while there is no rushing of air over the cabin, you can feel and hear (slightly) a rumbling maybe, but the usual jet noise of being outside and having a jet taxi is almost non-existant. I'm not saying you didn't hear something, but in other situations with being in a cabin, the attenuation of noise is great and I'm thinking you heard something other than the jet. Different story at an airshow being in the open, on the ground, with military guys and gals trying to make a lot of noise with aircraft with little noise abatement laws about their engines.
I haven't seen it lately but one of the big newspapers has a section called ask the captain or something like that, and if I see it again, there is usually a section to write in. I'll ask how close other planes are when they cross at minimum, and also about hearing another plane.
Interesting question that I've not given thought to before, but don't think you can hear it, nor for more than a few milliseconds at best at those crossing speeds and open air.
Dale