Personally I would strongly recommend against having both Mercurial and Subversion VCS. Aside doubling administration time for management different VCS, it mostly negates any benefits of using Mercurial in a first place.
I have been through similar path before - working locally in Mercurial whilst officially there was only SVN repo. Well, I got used to flattening revision history to get in more digestible by Subversion but I was not anything close to pleasant experience.
Once we eliminated Subversion out of the equation my life started to be significantly happier.
Probably with small projects the pain in not that noticeable but having co-existing source control system just add up unnecessary development workflow steps I would avoided.