Dear Client, Thank you for also contacting me with some questions about the demolition of the party wall, and tax upon property for your two flats. In most instances, partially divided walls can be demolished, provided that such works will not have the effect of impairing the structural safety of the building.
But then again, it would be perfect if you are able to consult the builder or a qualified structural engineer for the confirmation that this wall is not load-bearing. Sometimes, you would even require NOC from the housing society and other concerned authorities so that your local building regulations are in good book, and you do not get into disputes.
Now, property tax you have two separate agreements for two flats therefore BMC will treat these two as two units only.
Since the exemption is on residential units of less than 500 square feet each you can continue getting this exemption for both flats though BMC re-designates it as a single unit and gives it a composite area of 660 square feet.
So, before issues come up, right would be that BMC clarifies properly, so that any structural change gets all the necessary approvals.
Hope that this helps you to solve your problem.