Auditor with different entities - expert pls help !

This is also acceptable and some arrangement has been made. A company ( a CPA firm ), not a limited company, is an auditor duly appointed by members in AGM. A is a practising CPA duly registered under the Professional Accountants Ordinance and the name is entered in the list of CPA firms with members holding practising certificate issued by HKICPA.
B ( a limited company ) could be any person holding out the capacity as your company's tax representative. There is no requirement that this tax representative must be a practising CPA. B is only handling the taxation affairs of your company. You can check the engagement letter signed between B and your company. Moreover, there is a tendency that the auditing and taxation services be segregated for 'inpendence' issue.

HKICPA prohibits auditors from providing other services which may compromise their independence position. Your auditors should be a sole proprietorship or a partnership, therefore it will not appear on the CR list but must be registered with HKICPA because otherwise it cannot perform audit. It is a common practice amongst CPA to use a limited company to perform secretarial, taxation and accountancy work, and there is no surprise that your auditors' name is not the tax representative. In usual case, you should choose your auditor using big 4 (if you can afford them!) or by referral. That will be much safer.

As you mentioned, ' Company A ' can appointed more than one auditors in the same Company.

Just like one hospital have many doctors!!
i.e. such auditor maybe an employee of the ' Company A '.

Therefore, there is no informations in CR.

Don't be worried ! All the auditor must have professional ethics.