I don’t have the exact configuration changes you need to do to make this happen, but it should be possible.
All the services are enabled in the xml configuration files. If you don’t want the Admin modules to be accessible at all through your frontend web site, you can remove all the references to Admin from the xml files. Then Magento won’t know that it exists, so no one should be able to get to it.
I think there’s also a “disable frontend” flag in the xml configs somewhere, so you can make an install that has *only* the Admin enabled.
As long as both installs share the same Database configuration, they should manage the same data. The rest of the configuration would depend on how you have your web servers set up.