Fireplaces, Heating Stoves, Flues and Chimneys - Help with a shared flue

I have recently discovered that my furnace and fireplace share the same flue.
The furnace/water heater is in the basement and the fireplace is on the first floor. They have separate flues until they join at the first floor ceiling. Then it is one flue from the second floor through the crawl space and out the roof.
Here's my question... Can I run 2 liners down this one flue (assuming they'll fit) to essentially create 2 flues?
I checked the 2003 IRC and section R1001.9.2 states "Space around lining. The space surrounding a chimney lining system or vent installed within a masonry chimney shall not be used to vent any other appliance. EXCEPTION: This shall not prevent the installation of a separate flue lining in accordance with the manufacturer installation instructions."
This sounds to me like it can be done but I'd like some other opinions. The house is about 90 years old and the chimney is masonry with a clay lined flue.
My second question would be, if it can be done and there isn't enough room, could I break out the clay liner to gain more room for the liners?

Thanks for any and all help,
John

Chances are you will not be able to run the chimney liners as your envisioning. Between the furnace and water heater alone you will likely need a minimum of a six inch flue (depending on the specs of each).

Additionally, it is unsafe to use the current setup. While gas and oil burning devices can share a flue, a wood burning devices has to have their own.

Depending on the area in the chimney cavity you may be able to break out the existing lining and install appropriate sized stainless liners for each of the appliances. I would start by trying to figure the area in your chimney cavity and go from there. Good Luck!