a.js

// A. Create a new object that will contain the names ES6 module exports. We create it immediately so that
// it will work across cycles.
var es6Out = {};

// B. Assign it immediately to module.exports so other files that try to require this file will see it
// right away.
module.exports = {
  __es6_module__: es6Out
};


// C. Require another module, and check to see whether it has an ES6 module on it. Because the other module
// also created its ES6 module eagerly, this module will be able to get a handle on an empty copy of the
// soon-to-be-filled-in ES6 modules in the cycle case.
var _b = require("./b"), es6Module = _b.__es6_module__, b;

// D. If we find an ES6 module, save it off for future use. Otherwise, use the regular node module.
if (es6Module) {
  b = es6Module;
} else {
  b = _b;
}

var exports = function() {
  // E. Rewrite bindings that came off of `import` statements to lazily get their values from the module.
  console.log("In a. bar = " + b.bar);
};

var foo = 'foo';

// F. Fill in the ES6 module, including default export to point at the exports function.
es6Out.default = exports;
es6Out.foo = foo;

// G. Stick the ES6 module onto the standard-looking node export.
exports.__es6_module__ = es6Out;

// H. Replace the temporary module.exports with the standard-looking node export.
module.exports = exports;

b.js

// This works the same way as the description in a.js

var es6Out = {};
module.exports = {
  __es6_module__: es6Out
};
 
var _a = require("./a"), es6Module = _a.__es6_module__, a;
if (es6Module) {
  a = es6Module;
} else {
  a = _a;
}
 
var exports = function() {
  console.log('In b. foo = ' + a.foo);
};
 
var bar = 'bar';
 
es6Out.default = exports;
es6Out.bar = bar;
 
exports.__es6_module__ = es6Out;
module.exports = exports;

c.js

var a = require("./a");
var b = require("./b");

a();
b();

/**
  Output

  In a. bar = bar
  In b. foo = foo
*/

If we want to make regular Node module objects available as the default import (see my comment), we'd presumably be doing this:

--- a/a.js
+++ b/a.js
@@ -18,7 +18,7 @@ var _b = require("./b"), es6Module = _b.__es6_module__, b;
 if (es6Module) {
   b = es6Module;
 } else {
-  b = _b;
+  b = { 'default': _b };
 }

 var exports = function() {