[paths]section of the repository configuration file. This file is in .hg/hgrc, and you would add paths like this:
default = http://your-default-repo.com/repo
Then you would run
hg pushto push to your second repo and
hg pushto push to your default repo.
Unfortunately its not possible to run hg push * to push to all remote hosts at once, you have to specify
each push destination one by one.