Not easily - you would have to keep each one in sync with the other, via a multitude of hook and event scripts. It is possible, for example, to checkout from subversion, do an hg init
in that folder, and temporarily work using mercurial, and then eventually commit back to subversion when you're happy (ensuring you don't add and commit and .hg
files). Also depending on the number of developers you could get into all sorts of problems with conflicts from merges if two developers commit to the same file under different version control systems.
Since Subversion (and CVS) are centralised version control systems, and git, bazaar, and mercurial are all distributed, keeping them in sync would require choosing a master copy for each one.
The VCS tools you mention are all fairly similar on the command line (though CVS really shouldn't be used any more), so the learning curve for other users isn't particularly steep.