"When silencing parts between vocal lines, please keep in mind that a sudden drop in background ambiance can have an bad effect, so at the very least fade the area around the silenced part, to minimize that effect. Rules to start with are, fade in quickly and fade out slowly. Alternately, use the envelope tool to lower the volume in that area. That way, you can comfortably change it later." - Audacity  The envelope tool doesn't make a whole lot of sense to me. Yet.

"When you're satisfied, fade out the last two thirds of the overlapping upper part of the track, and fade in the first two thirds of the lower overlapping audio." - Audacity  - emphasis mine.