Share data between two DLLs that are called by Main program

I work with a finite element software that allows user-defined functions loaded from DLLs, one DLL per each function. 

My two functions share the same module, but when turned into DLLs and loaded by the main program, data between is not shared between the two DLLs. 

Examples that I have seen, like here, are all about sharing data between DLLs and the main program. I was wondering if it is possible to share data between two DLLs that are called by the main program. 

And please note that the main program is a black box to me and I have no control over it. 

Thanks,

Alireza