consider this code:

// a
import { f_b } from 'b'
console.log(f_b)
export function f_a() {}

// b
import { f_a } from 'a'
console.log(f_a)
export function f_b() {}

The best I can do using the ThirdPartyModuleRecord is this:

const a = {
    bindings: [{ export: "f_a" }, { import: "f_b", from: "b" }],
    initialize(env) {
        function f_a() {}
        env.f_a = f_a
        console.log(env.f_b)
    }
}

const b = {
    bindings: [{ export: "f_b" }, { import: "f_a", from: "a" }],
    initialize(env) {
        function f_b() {}
        env.f_b = f_b
        console.log(env.f_a)
    }
}

And SystemJS format can emulate this behavior:

System.register([
    "b"
], function(_export, _context) {
    "use strict";
    var f_b;
    function f_a() {}
    _export("f_a", f_a);
    return {
        setters: [
            function(_b) {
                f_b = _b.f_b;
            }
        ],
        execute: function() {
            console.log(f_b);
        }
    };
});

There are 2 stages in system js, the hoisted function declaration is set in stage 1, and the actual runtime code is run in stage 2 (execute).