"define varibles like char data char1 _at_ 0x40; ... in C file,but i don't know if it will work since these bytes are double defined."

You're right - it probably won't work for that very reason.

Even if it did work, that approach is best avoided as you would have the same addresses defined independently in two different files: sooner or later, someone is going to update one & forget the other - leading to all sorts of obscure, hard-to-debug problems!

Erik has given you the principle you need to follow - it is exactly the same principle that you'd use to share assembler variables between assembler files, or 'C' variables between 'C' files; in fact, it is the general principle used to share any symbol between modules!