I would reconsider whether or not you're ready to split it.
In my current situation, I have continuing the development of an application, which is also in production.
I have split it into three sections, the front end, and two backends, where one backend contains the live data files, and a second which contains system tables, where system tables are those tables needed by the application (ie meta data and system level data), but not ones which would be updated in normal production. The users, at least for now, are using a single shared front end, which is linked to the two backends. I have a local copy of the front end, also split, but I also have local copies of both backends. This allows me to develop locally with test data, and provides me the ability to create and modify the system tables locally as necessary. When the time is right, which right now is about twice a week as we are heavily into development of new functionality, I update the shared front end and system backend as necessary, reset the links, and proceeed. I can also, if necessary, relink my local front end to the production file for testing if something seems awry with the live data.
Once things stabilize and updates can be scheduled, then the shared front end will become local to each users, but both backends will of course remain shared. That will require more effort for front end updates, but that process is also being automated.
I'm also curious to know what techniques others are using as well. It is an ongoing problem dealing with production systems and continuing development.
Good Luck
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As a circle of light increases so does the circumference of darkness around it. - Albert Einstein