IMHO this is fine if the two files contain disjoint sets of information.
If the information overlaps, that raises the question of why not just
auto-generate one from the other.
I'm not as worried about duplication as you are, but perhaps that's because I'm only thinking about SRFIs, which are effectively set in stone once they are finalized.
Could we come up with the most hairy example of HTML markup possible? As
in, this is the worst it's going to get. Then we would have a data point
to discuss :D Maybe some macro involving optional and rest parameters as
well as quoted parts.
That sounds great. You could make something up, or we could all look for the worst example we find while working on our examples from existing SRFIs.