Dear Mr.Sudarshan
I concur with the view of Mr.Udit Sharma. The merger of two partnership firms (audit firms) cannot be construed as a casual vacancy caused by the resignation of the auditors. Hence in my view the board of directos has the powers to appoint the merged audit firm (in your case X & Co) as statutory auditors until the next annual general meeting.
For your kind information I give below a note of a public limited company given to national stock exchange. the company has faced the similar situation. the board of the company had appointed the merged entitity as its auditors until the next agm. The website link is also provided for your information.
Dynacons Systems & Solutions Ltd. has informed the Exchange that the Board of Directors of the Company in their meeting convened on March 10, 2010 have considered and approved the appointment of M/s. P.C. Ghadiali & Co. to hold office as Statutory Auditors of the Company for the period January 01, 2010 until the conclusion of the next annual general meeting of the Company in place of existing auditors M/s. Soni Palan and Associates, due to Casual Vacancy arising out of dissolution of their firm resulting from the merger of two Firms of the auditors, namely M/s. Soni Palan and Associates and M/s. P.C. Ghadiali and Co.
https://www.nseindia.com/marketinfo/companyinfo/eod/announcements.jsp?symbol=DYNACONS
with regards
Muralidharan