How I'd do code splitting in Redux (pseudo code, not tested!)

reducers.js

import { combineReducers } from 'redux';
import users from './reducers/users';
import posts from './reducers/posts';

export default function createReducer(asyncReducers) {
  return combineReducers({
    users,
    posts,
    ...asyncReducers
  });
}

routes.js

import { injectAsyncReducer } from './store';

function createRoutes(store) {
  // ...

  const CommentsRoute = {
    // ...

    getComponents(location, callback) {
      require.ensure([
        './pages/Comments',
        './reducers/comments'
      ], function (require) {
        let Comments = require('./pages/Comments').default;
        let commentsReducer = require('./reducers/comments').default;

        injectAsyncReducer(store, 'comments', commentsReducer);
        callback(null, Comments);
      })
    }
  };

  // ...
}

store.js

import { createStore } from 'redux';
import createReducer from './reducers';

export default function configureStore() {
  let store = createStore(createReducer());
  store.asyncReducers = {};
  return store;
}

export function injectAsyncReducer(store, name, asyncReducer) {
  store.asyncReducers[name] = reducer;
  store.replaceReducer(createReducer(store.asyncReducers));
}

Just in case someone is interested.
I have implemented a running ssr example for next.js.

vercel/next.js#1459

Here's a solution for injecting the initialState in async reducers, suggested by @gaearon
https://gist.github.com/cvbuelow/ea7bd4deb7824fdb65d81cd2b8c60f5e