Not sure how those with the shared neutral work in GFCI circuits...
An AFCI isn't the same thing nor will it provide the desired protection as a GFCI. The AFCI detects arcs that could cause fires and then trips. The GFCI detects leakage current that might be going through you that could kill you.
I suppose the fire could kill you too, but that's a different story.
Might be time to run two new 12/2 circuits. Feed the first outlet with a GFCI recepticle in each branch and you'll be good to go.