Not quite sure what you are trying to achieve, but you can really only have one fax machine attached to an incoming number. An untested way off the top of my head may be to put two analogue extensions in to a ring group, and set it to cascade ring, but i'm unsure how it would handle things as they are analgoues. This is completely untested, and only an idea off the top of my head though.
You could then set the outbound CLI of both analogue extensions to display the same number. That's the easy part.
A much better solution would be to have a dedicated fax server such as Nupoint or GFI.