Sure you can. On the left hand side of the timeline, where you see the track labels (V1, V2, V3, etc… & A1, A2, A3, etc…), you’ll also see a button labeled v1 (note it’s lowercase) on the video tracks, and a1, a2, a3 etc… on the audio tracks. You can move these buttons up and down to select where you want video or audio from the viewer to end up. You can also simply click them to either allow (when for example the v1 button is touching the V1 label) or prohibit (when they are not touching) that track from receiving the corresponding track in the viewer when you place that clip in the timeline.
I hope that makes sense, and I’m sorry I don’t know the official terms for what they’re called, but that’s how you do it.
Best,
-Grant Gomm
http://www.grantgomm.com
EDIT: This works on FCP 7 and below, but I have no clue about FCPX if that’s what you’re using.