Click on the event fX icon for the event you want to mirror, and choose the "Sonic Foundry MIrror" fx. You can reflect vertically, horizontally, or at an angle, and you can choose to put the reflection plane anywhere.
I used this in a volleyball video last fall. I had filmed the team warming up. As part of the warmup, the coach tosses balls straight up, just in front of the net, and they run to the net and spike the ball. I put the camera on the ground, at the net, and filmed the action. I then applied the mirror effect and moved the plane of the mirror until the net just disappeared. I used the glow effect, and a little slow motion. The effect was absolute magic as they came to the net, seemingly from both sides, spiked the ball, and then magically disappeared (they duck under the net when they are finished).
Cool shot. Did you have that effect in mind when you shot the footage?
Nope. When I start a project like this, I take some of the footage and start trying various ideas. I can't remember how I happened on this one, but it was not anything I had seen somewhere else. I just kind of stumbled on it. If you are interested, I just put the video back on my web site for a few days. The effect happens about one minute after the start. It is WMV format at 256K, so the quality is pretty awful (Real Video is better, but too many people complained that they didn't have the player).