mutually dependent binaries

a.cpp

#include <iostream>

__declspec(dllimport) void b();

__declspec(dllexport) void a()
{
  std::cout << "a()\n";
}

void calls_other_b()
{
  std::cout << "calling b()\n";
  b();
}

int main()
{
  calls_other_b();
}

b.cpp

#include <iostream>

__declspec(dllimport) void a();

__declspec(dllexport) void b()
{
  std::cout << "b()\n";
  std::cout << "calling a()\n";
  a();
}

b.def

LIBRARY   b
EXPORTS
   ?b@@YAXXZ @1

script.cmd

% generate b.lib
lib /def:b.def  

% build a.exe and produce a.lib
cl.exe a.cpp b.lib

% build b.dll
cl.exe /LD b.cpp a.lib

a.exe