Have you tried it? I'm going to guess that if this is a "web" database, then it cannot link to anything except SharePoint tables. My understanding is that you can create a standard client/server app and link that to what ever you want including SharePoint, SQL Server, Oracle, ACE, all at the same time. But, you will distribute the FE as you would any FE. It doesn't run under the control of SharePoint and it isn't published.