As far as I know, this is not possible. The basic unit is “single document”. For good reasons.
How do you preview a “composite document”? How do you search it? This just does not work. If you forego these requirements, you could as well zip these docs into an archive and file that. For me that would not be useful. The paradigm with DT, I’d say, is that you can easily search and preview. So there must be a notion of a clean file type.
Actually, RTFD files do in some sense possess the ability you look for and also illustrate what the problems are: In RTFD, you could make one RTF wrapper doc, and then embed arbitrary amounts of files, pdf, ppt, you name it. Downside? They will not be searchable, and do not exist on their own. Right now, you might feel that you only want these docs (presentation and code) as a single unit. But soon enough, you will want to replicate the code part into groups that categorize certain code (presentation A’s code will get replicated into group “Python examples”, and B’s into “Ruby code”). In the “one unit” scenario, you’d have to drag the presentations along. This will not work in the long run.
Options come to mind:
Make an RTF (or markdown document) that is the “single unit” for that event. It contains x-DT-item links to the relevant materials (talk, code snippet(s)) and can then also contain notes on the presentation etc. The actual presentation files and code files will be filed separately. Downside: the link between the items is DT-specific. And: the are no automatic backlinks from these docs back to the RTF “anchor doc”. If you hit upon them, e.g. in a search, independently, you would not know what anchor doc they came from and who their siblings are. But you can add such links manually, e.g. in the spotlight comment box.
This would work very well if we had an additional field in DT where we could add rich text notes, including clickable links, to ANY document. Spotlight comments are plain text only (which makes them very portable, though), and if I put links in there, I have to use detours (like highlighting the link with the mouse and selecting “open URL” in Services).
Put of reference between the presentation and the code file in their respective spotlight comment box. This will survive export from DT (but not necessarily from OS X).
Make a group the anchor for the specific event and replicate all related docs (presentation, snippets, notes) into that group. That’s of course, what we would do in a Finder environment in the first place. In DT, I personally would not like that too much, because of the proliferation of groups. I like to use them mostly as (reasonably sparse) categories.
It seems to me that one could include relationship information in the metadata for each component document of a collection such as is being discussed here. It would be a bit awkward, but would still provide the interconnections within each component that would allow one to recreate the full bundle.