As you are an unregistered association, you can proceed as follows:
1. All members on your side should agree. You call a meeting of all members on your side. Your notice should contain the following agenda: (1) Formation of the association and adoption of a name and a memorandum of association (2) Election/co-option of a provisional committee with Chairman, Secretary, Treasurer and others. (3) Subscripttion from members to meet the initial expenses of formation and registration of the association and also maintenance until final registration (4) Opening of a bank account. You find out from the manager of the bank where you want to open account the requirements for opening account in the name of the Association. If the requirements cannot be met immediately, you open a joint account in the names of the provisional committee members.
2. After you have decided as above the provisional Chairman or Secretary should write to the other group regarding your decision and invite objections if any. If the Chairman and/or Secretary of the existing unregistered association is/are from the other group you can write to him/her. Or you may have to write to all members individually and invite them for a joint meeting of members from both the buildings. If members from the other building give unreasonable or arbitrary reasons you can ignore them. One main question would be bifurcation of common assets. You said that water tanks are on top of your building. You will have to amicably solve this problem. There may be other common assets also.
3. When a reasonable situation is reached you proceed to register your Association. In my opinion registration under the Karnataka Ownership Flats Act would be the best. You may appoint a competent lawyer or an experienced person from one of the existing registered associations in Bengaluru as your consultant.