How to combine two projects in Mercurial?

I was able to combine my two repositories in this way:

1. Use hg clone first_repository to clone one of the repositories.
2. Use hg pull -f other_repository to pull the code in from the other repository.

The -f (force) flag on the pull is the key — it says to ignore the fact that the two repositories are not from the same source.

Here are the docs for this feature.