Lateral conversion of two flats into one - implications

General Discussion

Hi All, I manage two first floor flats in adjoining semi-detached blocks. Each flat is owned by different Companies (both of which I have control of).
I am considering knocking through from one to the other to make a much larger flat on one level.
I will inform the Council so only one council tax bill is issued. Utilities will be kept separate in case we ever divide the flat back into two.

Do we need a written agreement between the companies?
What consents will be required?
What are the implications? Do I need planning permission? We are not touching anything externally.
How do I apportion the rental income and expenses?

Would a Tenancy agreement reflect two addresses or just the one with the main entrance.

Any thoughts gratefully received.