cjs.js

global.fn = d => d + 1;
module.exports = global.fn;

output.js

// chunk1.js
define(['exports'], function (exports) { 'use strict';

var commonjsGlobal = typeof window !== 'undefined' ? window : typeof global !== 'undefined' ? global : typeof self !== 'undefined' ? self : {};

commonjsGlobal.data = [4, 5, 6];
var shared = commonjsGlobal.data;

exports.* = commonjsGlobal;
exports.default = shared;

});

// thing1.js
define(['./chunk1.js'], function (__chunk1_js) { 'use strict';

__chunk1_js.*.fn = d => d + 1;
var cjs = __chunk1_js.*.fn;

var thing1 = __chunk1_js.default.map(cjs);

return thing1;

});

// thing2.js
define(['./chunk1.js'], function (__chunk1_js) { 'use strict';

var thing2 = __chunk1_js.default.map(d => d + 2);

return thing2;

});

rollup.config.js

import commonjs from 'rollup-plugin-commonjs';

export default [{
    input: ['thing1.js', 'thing2.js'],
    output: {
        dir: 'out',
        format: 'amd'
    },
    plugins: [
        commonjs()
    ],
    experimentalCodeSplitting: true
}]

shared.js

global.data = [4, 5, 6];
module.exports = global.data;

thing1.js

import e from './shared';
import fn from './cjs';
export default e.map(fn);

thing2.js

import d from './shared';
export default d.map(d => d + 2);