What are your preferred techniques for dealing with characters who leave a scene before the shot is over? I've found that it seems to dramatically increase my tracking time when one of the characters is totally out of view of all the cameras.

Thanks for pointing out this issue. We'll look into ways to prevent such degradation of performance.

I recommend you the following solution:

1. Track project with 2 actors until one of them leaves the scene, or you simply do not need to track his motion further.
2. Copy the pose of staying actor in the last tracked frame to the clipboard.
3. Save the pose from the clipboard to a file.
4. Create a new project for the same video with a single actor.
5. Load saved pose from a file to the clipboard.
6. Go to that frame number an paste the pose.
7. Start tracking from this frame.

I suppose you can easily chain animations for staying actor exported from both projects in your animation editor.
Commands for working with poses and the clipboard are located in the Edit menu.