如何让多个ajax调用在redux-observable中并发？

import { mergeMap, map } from 'rxjs/operators';
import { ajax } from 'rxjs/ajax';
import { ofType } from 'redux-observable';

const multipleAjaxEpic = action$ =>
  action$.pipe(
    ofType('MULTIPLE_AJAX_REQUEST'), // 监听特定的action类型
    mergeMap(action =>
      // 使用mergeMap操作符将多个ajax调用合并为一个Observable
      ajax.getJSON('url1').pipe(
        map(response1 => ({
          type: 'AJAX_SUCCESS',
          payload: response1
        }))
      ),
      ajax.getJSON('url2').pipe(
        map(response2 => ({
          type: 'AJAX_SUCCESS',
          payload: response2
        }))
      ),
      // 可以继续添加更多的ajax调用
    )
  );

import { createEpicMiddleware, combineEpics } from 'redux-observable';

const rootEpic = combineEpics(
  multipleAjaxEpic,
  // 可以添加其他的Epic函数
);

const epicMiddleware = createEpicMiddleware();

const store = createStore(
  rootReducer,
  applyMiddleware(epicMiddleware)
);

epicMiddleware.run(rootEpic);