Is it legal for a large company to get it's financial statements audited by more than one audit firms?

one audit is sufficient to audit the company. The important is the Audit firm MUST be internationally qualified according Standard Accounting Practice.

As per International Accounting Standards and applicable corporate law, only one auditor is requred to give an opinion on the finacial statements of a company for the statutory audit. Accountants are bound by a code of ethics which prohibits such a scenario unless the second audit is not the minimum set by law for an annual statutory audit.

However, an entity can engauge several auditors to give opinions other than that of a statutory audit. eg. other aspects of oparation e.g. fraud investigation.

Its depends upon the nature of the company's operation and services which it obtain from its audit firm, if audit firm not specialist in one area of operation, then company can got the services of other audit firm in this scenerio. 

Usually in parant and subsidiaries companies causes, companies used different audit firms

In case of Audit of large Firms with multilocation world over/within the country/within the region which is having100s of1000s of branches, One Single Audit Firm/or even a few number of firms, it is practically not possible to perform all the Audit functions. Under such cases there will One lead Auditor and he will hold the right to Certify the Consoldiated Audit Reports.  Lead Auditor will have the right to fix certain parameters for the Audit and to be maticulously followed by the others in the Team all over.  Two/more Lead Audit Firms cannot perform Audit in case of a legally appointed Lead Auditor without delegated authority from the Lead Auditor.  

This is done in companies, international companies and multiple entities

Yes it is already at multinational companies

No, one Audit firm is enough to get audited..