> For large sites, I will often break up a mirror into
> smaller sections. For instance, consider a forum
> with a very active "tech" section, but where the
> other sections see a lot less activity or are of
> less interest. I will have the tech section set up
> separately so I can update the mirror for that
> section more often and run the update for other
> sections less frequently.
Updating is fast, I get approx 3 files per second, size independent. The only
exception is dynamically created content is always redownloaded.
Also, all support files, CSS, JS, images will be duplicated on separate
mirrors. Thus overall, the mirroring takes longer.

> What I would like to know is if there is a way to
> merge two mirrors. 
No. Any files added would not be known by HTT (Not listed in the cache) and
thus deleted at the end of the Update.