Redrawing the curves, tracing the artwork, would be the easiest method in my opinion.
It's been a looooooooong time since I had a system which could run CS5 apps, let alone since I used Fireworks. However, tracing the sides with the Pen Tool in Illustrator is fairly straightforward.
Note it is not necessary to redraw the sides. One could easily just copy the artwork then delete everything except the path for one side. Then repeat for the other side in the copy.
This works, but in doing so the Blend method below for the centerline will fail due to the construction of the original paths. So you'd have to determine the centerline yourself. (This is why I'd choose to merely redraw the side paths.)
Then, with Illustrator I merely created a blend between the sides to get a center path...
Object > Expand the Blend to get that centerline as an actual path and
Edit > Copy it
Object > Path > Join the bottom and top anchors for one side and you then have a shape for that side.
Edit > Paste in Front to place the centerline again
Object > Path > Join the bottom and top anchors on the other side, and you've got a shape for each side...
I can not remember how to do any of this in Fireworks at this point, other than drawing with the Pen Tool.
(While not Fireworks CS5 specifically, this all uses Illustrator CS6... which is as close as I can get to FW CS5.)