Even with several sites, you can still use copies of the same source, provided you have a build process for deployment of these many sites which uses the source code from the same place in your version management system.
You wouldn't then need to let them become out of sync any more than necessary, your build system can keep track of what version is deployed on to each site, and when you do new deployments, your build script could automatically pull the latest one out of source control.
Or something.
Anyway, think about it. It's only one file.
Having parts of the source of a web app updated at different times sounds like a recipe for disaster- it's important to be able to thoroughly test after each update, even if this is only of a component.
Loading the file at runtime from another server is definitely a bad idea.
Mark