> If I have mirrored one site to a certain depth, then 
outside 
> of HTTrack manually follow a link therein to another site 
on 
> the Internet that I now wish to mirror as well, how can I 
> have the two mirrored sites know about each other on my 
> local system?
Re-launch WinHTTrack, select the existing project, 
choose "Continue an interrupted mirror" option, and just 
ADD the desires URL(s) to the existing URL(s).

This will append the sites to the existing project.