Building XDC Modules and Specifying Data locations

You can do everything in the .cfg file, but that means you'll have to repeat these steps in every .cfg file that uses those modules. There are many ways how you can do it, and here is one of them. Please let me know if some assumption I made does not work for you.

Let's say ModA owns the buffer. This doesn't change much because both modules will access the buffer through their own pointer, and the memory allocation is done by XDCtools.

ModA.xdc:
================
module ModA {
    config Char bfr[];
    ...
}

ModB.xdc:
================
module ModB {
    config Char* bfrA;
    ...
}

app.cfg:
================
var Memory = xdc.useModule("xdc.runtime.Memory");
var ModA = xdc.useModule("buffers.ModA");
var ModB = xdc.useModule("buffers.ModB");
ModA.bfr.length = 34;                                        // set the buffer size
Memory.staticPlace(ModA.bfr, 0, ".myBuffer");                // allocate the buffer to a separate section ".myBuffer"
Program.sectMap[".myBuffer"] = new Program.SectionSpec();
Program.sectMap[".myBuffer"].loadSegment = "L1DSRAM";        // allocate ".myBuffer" to L1DSRAM
ModB.bfrA = ModA.bfr.$addrof(0);                             // set the pointer in ModB to the buffer in ModA

After you set up all this, you should be able to access the buffer through ModA_bfr in ModA, and ModB_bfrA in ModB. If you wanted multiple instances of ModA and ModB, you would create the buffer and the pointer to it as instance config parameters.
Here are the links for more information about Memory.staticPlace and use of $addrof operator.