Is a multi wire shared neutral circuit a 220 volt circuit
You could use a subpanel. 

I'm assuming you have a series of 120V receptacles on this circuit, at 20A or 15A. 

You would keep the 20A 2-pole breaker on the main panel, and the 12/3+G cable. 

At the other end, put a very small load center. You can use a "main lug" type, 2-circuit, 4-space. They're under $20. For example: http://www.amazon.com/dp/B000BPOROY/

Then you can put AFCI breakers in this subpanel to branch out as needed.

I haven't gotten a reliable answer as to whether 20A subpanels are allowed in code (30A may be a minimum), but the electrical theory is sound.