Sandro, there seems to be an issue, as to whether there will ever be a two-pole combination type AFCI. I have seen a memo, purportedly from NEMA (note that I am still not convinced of it's authenticity), that essentially says "too bad" if the lack of two pole AFCI's eliminates the use of multi-wire branch circuits.
The 2008 NEC also introduces plenty of restrictions on the use of the "shared neutral." It's almost as if they want to discourage the practice.
On the bright side .... there is an opening for AFCI devices. While the code still calls for the run to the panel to be in a metal raceway, IMO device AFCI's are ideal for old work, such as you describe.
