Dual Employment - Appointment Letter & Compensation

I am working in a Pvt Ltd Company A for 16 years. My boss is the Managing Director of Pvt Ltd Company A and Pvt Ltd Company B. Manager from Company B resigned so my boss asked me to handle both companies. Need a fresh appointment letter to be drafted as I will be drawing enhanced salary from Company B (Salary of Company A + B from B). Help pls...

Now I believe management will issue a letter of additional responsibilities and salary increment. Responsibility will include for managing both the company.

Dear Govind Sing,
I am moving on to managing 2 pvt ltd companies (A & B) and henceforth salary for this combined task would be credited cumulatively from Company B only. Internally Company A will reimburse their portion of Payment to company B.
I need this transition to be properly documented to reflect the fact that this is a promotion to a bigger role. Of note is that I have been a Consultant in Company A and will be moving on to being an employee of Company B by accepting the new role.
Pro's and Con's?
Should I exit from Company A and enter Company B or is there a better alternative?
What letter should be obtained from Company A and Company B to make this transition meaningful and complete in all respect?
Can I hold a joint position of two separate pvt ltd companies legally by being part of one? If yes, how?
Thanks for all the help in advance.
Regards,
Raj

Dear Raj,
You may go for a tripartite agreement signed by A, B & yourself. Mention all T&C in the letter like transfer of your PF, gratuity, continuity of services, superannuation and other dues from A to B.
Also incorporate your dual responsibilities (specifying principal employer and secondary employer as a visiting consultant)and revised compensation.
Make sure your PF & taxes gets deducted from B and you get form 16 from B only.
Rest you should not face any problem in handling the daily task as it is like shared services job.
Regards
Haider