1. If there are two owners, then both have to initiate the process jointly.
But regular follow up by at least one should be there.
2. You will not get responses to your frequent emails.
You have to resort to legal process once you dont get proper response to your personal efforts.
Consult a lawyer and send a proper legal notice demanding the reliefs.
3. First of all ascertain the facts of the case as well as other circumstances.
Dont jump into any conclusion hastily.
The details would be different from the builder side to that of the banker.
The banker's demand is for repayment whereas the builder's demand would be of a different subject.
You should ascertain from the tripartite agreement that who should pay the amount to bank and how much etc.
4. Your entitlement of super built area would have a mention in the agreement itself, if not you can demand the details of the same from builder.
If you find that the builder is violating the agreed conditions you can make a representation tothe builder and seek his explanation after which if you still are not satisfied, then you may resort to legal process.
5. If the builder has to reimburse to bank directly then why would the bank ask you for the repayment,
Consult a local lawyer and decide about further action as per his advise.