Strangest thing -- when I delete either audio track, the remaining track sounds clearer

This isn't really a problem, because this weird deleting thing actually solves the problem, but it's the strangest thing I've ever encountered in FCP (and that's saying a lot), so I was wondering if anyone had any idea why.

I have a bunch of interviews. Shot with lavaliere mics going through the camera. It seems it was shot on a normal SD camera. There's a couple interviews where the sound is particularly bad. One, for instance, shot from a distance sounds like they're using the mic mounted on the camera because the sound is so extremely quiet.

Then, when I deleted one of the two audio tracks (after putting it in a sequence), it came through loud and clear. So of course I assumed the two tracks were two different mics (one the clear lavaliere and one the far away camera mic) because that makes sense. But no. If I delete either of the tracks, it doesn't matter which one, the sound goes from extremely quiet (around -42) with a lot of background noise to loud and clear (-12), with no background noise.

Weird right? Is there any possible explanation?

FCP 6.0.3/Mac G5 OS X 10.4.11