By default, the audio output of Final Cut Pro is stereo. You can get it to output split tracks, but you’ll need to do the mix in Soundtrack Pro or ProTools. This tutorial explains how.
A while ago, I wrote about how and why to create submixes in Soundtrack Pro. While submixes can be used for reverb, a better way is to use Sends and Busses. This provides greater control and more flexibility than a Submix, but it is a bit trickier to understand.