Is your code arranged in the classic/normal 1 ino file, many cpp fies, and 1 h file included in every ino and cpp file that needs it using #pragma once? That way a global in the .h file is visible to all ino and cpp files/functions/procedures.
If you are attempting to use both cores of a multi core board so have 2 compile units, then I think adding extern to a variable in both compile units makes it the same piece of memory and accessible to both (NOTE: I have not tested this, just read about it)