Do Multiple Tanks Add Together for Capacity?

I have a project that I am looking at that right now and it is supplied by a private 75,000 gallon water tower for their current systems and domestic water supply. There's no city tie-in, just fed from their private well.

They are upgrading and adding sprinklers in other areas and will need a fire pump. The hazard classification will require 120 minutes of a water supply. That 120 minute supply is roughly just under 250,000 gallons.

Can I somehow tie into the water tower underground that comes into the building and connect a new tank that will supply the new fire pump to meet these requirements?

That supplies me with 75,000 gallons, so presumably I would only need another 175,000 gallon tank.

Or do I need a separate tank for the 250,000 gallons?

​I am not sure if the well pump into the tower matters or if that can be figured into the gallons needed as well. Looking for advice on how to approach this situation. Thanks in advance.

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