In a building, one cooling tower is on the top roof and the other is on a roof 20 M below the top roof. Will the system work as one central system and how?

Yes the system can be run as one central system because in cooling tower the system will be open when it is entered in the cooling tower because the water becomes free fall upon the entry and difference of height with base level.
The suction of condenser water pump will remain n the same header so there will be no problem to disturb the system and it can be run as one.

to work as a one system the cooling towers should be closed circuit type, as the normal open type cooling towers can not work as a one system because of the towers basin different levels.

Open type cooling towers could still work installed at two different levels.
Please think over it once again.
Most of the questions raised by me are for technical discussions and I shall furnish the answers at the right time.
Thanks, Naik