phase reversal between audio channels

I was listening to W1AW code practice on 40 meters using 2 slices tuned exactly the same, except one slice was on the left speaker the second slice was on the right speaker (also using a headset). Seemed to work well. I then changed the L/R slider to the center for the left channel and noticed a decrease in the volume which I could reverse by going back to the left position. I tried the right channel slider with the same result, a marked loss of volume. The next step was to move the slider all the way to the left to match the other slice. The result of both slices on the same audio channel (left of right) was the almost complet cancelling of the W1AW signal in the output.
I guess my question is, could there be a phase reversal (180 degrees) between the audio channels? And if there is, should there be?
Don't really know why I was listening like this but at the moment it seemed OK.
You guys are doing a great job, under very trying conditions.
Thanks. Al N4AB