Decide how much video you want per disc (if you're using single layer discs, I'd recommend no more than two hours per disc). Once you've decided, look for a spot at the appropriate time on your Timeline in FCP for the end of disc one (e.g.: around the two hour mark). Cut that video at that point and copy everything after that point to a new Timeline (with the same settings). Repeat the process until you have a separate Timeline for each disc's video. Add chapter markers if needed, then export each Timeline Sequence using Compressor.
In Compressor, use the appropriate DVD Preset for the length of video you decoded to use per disc. Once you have all videos compressed, author and burn each disc separately in it's own DVD Studio Pro project.
-DH