Splitting your AC database in two is not a good idea. While I could pull it off, there are a few things to consider:
First, your database will be in an unsupported state as far as AC is concerned. While I certainly command the respect of AC support (at least I did in the past), I also command their consternation. Having such work done to your database, even successfully, may put you at odds with support.
Second, sooner or later you're going to need that old data and now you're in a pickle. Now you have to recombine your databases just to pull a few reports or answer to a legal summons or something. So now you have a lot of unexpected expense and impatient people waiting.
What you really want to do is consider the following:
-- How old is your server? If you have an older machine, it may be time for an upgrade to some good SSD drives and a faster controller.
-- How big is your database? SQL Server Express has a 10G limit on database size. If you have approached that limit, it's time for an upgrade to SQL Standard which abolishes that limit.
-- How is your network? You should have gigabit switches everywhere, good cabling, and all network cards should be gigabit.
Those are far better options than what you are suggesting, more cost effective for the long term, and do not put you at odds with AC support or put you at the behest of any third party.
Feel free to PM me for further details.
JamesNT