If you are just trying it out, simply use them in parallel. Manually sync the two as needed.
You can get familiar with it somewhat on the small scale with a local (non-cloned) hg repo, but you won't get comfortable with hg until you've worked on it as a team. I wouldn't recommend keeping this split personality for very long since the history will tend to be littered with non-helpful commit messages like "big sync between hg,svn". I would also suggest developing only on the default branch in the dual-sandbox. If you want to experiment with something on a named branch, try it in a hg-only sandbox.
Start by converting your svn repo to hg
Use development "sandboxes" that contain both the .hg and .svn subdirectories. You can probably just copy the .hg directory from a cloned repo.
Ideally, the developers involved in the trial should be committing to both vcs's in sync, but ...
To manually sync from svn to hg:
svn update
hg addremove
hg commit
hg push
To manually sync from hg to svn:
hg pull -u
(svn add newfiles)
(svn remove gonefiles)
svn commit
Pick a new feature that may take a little while to develop (days to a couple weeks), then start doing it in an hg repo converted from your svn base. That will give you a taste of the powerful branching and tagging that svn lacks.
I think my team did it pretty well. We set a date for switchover (one month out). During that time some of the team tinkered with hg and got a certain level of proficiency. This was instrumental in peer help.
We did a lunch&learn on day zero of the switchover to mercurial. We all brought our laptops and played in the same repo, changing files, committing, pushing,pulling. People were willing to experiment because it was meangingless data. Start the repo with a single text file, tell people how to clone/push/pull the repo then tell them to change something (don't specify what). Let everyone work at their own pace. They will create new files,change files, create conflicts, resolve them ....
This approach requires that you have at least one person who is fairly familiar with branching, and merging to act as a facilitator.