So there are two approaches to publishing a feature service, neither of which seems like you'll be able to accomplish in your current architecture. If you were going the route of a hosted feature service in Portal, the data would be copied from your original source location into the ArcGIS Data Store registered to the hosting server. It then wouldn't be referenced from the original location, (and this is assuming that there wouldn't be a problem copying the data from two separate tables). Another approach is to publish a traditional map service with feature access, but that requires that the data reside in the same workspace.