Given a configuration where I have two publicly visible web servers hosting the public facing website and one internally visible server hosting the website and CMS, is it possible to configure the Mirroring Service to push from the CMS instance to both of the front-end servers?
It is possible.
To achieve this you need to configure each target site under CMS > Admin > Config tab > Tool Settings > Mirroring. (as a side note this guide is really good for getting mirroring configure.)
Then when you start the mirroring task, you should see each of your targets sent to the Mirroring Service as a job.