Mercurial How To Merge 2 Repositories that share a common ancestor but are not clones of the same repo

I am using hg-subversion, and I have 2 different hg repositories one from our svn trunk, and one from a branch of the trunk. I would like to link them somehow. At some point in the history both Hg repositories will be identical is there some way to join them?

In other words is there a way to relate the repositories from within Hg?

The technique I am currently using is to just export the second repository over top of the working copy of the revision they share, and then commit that working copy as a branch in Hg, but I lose the history this way.

Any advice would be great

You could try importing the two repos into one as if unrelated, then merging them. (When you say they share a common ancestor, do you mean that those ancestors have the same revision ID? If so there is a good chance that this will work well.)

hg clone repoA
hg pull -f repoB # may not need -f
hg merge

Can you re-clone the Subversion repository, giving the root of the Subversion repository as argument instead of trunk or one of the branches? I used hg clone svn+ssh://foo/bar/baz once, where svn+ssh://foo/bar/baz/trunk was the trunk and svn+ssh://foo/bar/baz/branches/quux was a branch, and in the end I had two named branches in Mercurial, the "default" branch representing Subversion trunk and the "quux" branch representing the like-named Subversion branch.

If you have outgoing changesets in your existing hg repos, you might be in a bit of a bind there. If there are just a few outgoing changesets, it might work to enable the mq extension and to convert the changesets into patches. These patches can then be re-applied on the new clone (in the appropriate named branch) and eventually pushed to Subversion.