Its not mandatory to take their plan but I don't understand where 2nd tv coming into picture..if you select lets say ₹200 worth of channel , balance will be adjusted in future.. ultimately its your money! When you are paying initial amount, the are allocating some balance to dth and airfibre, depending on channel selection it will get deducted in subsequent months bill.
So even if you add a different connection to the Airtel black, you should get ₹100 off and ncf of ₹92... But this change will affect existing airfibre bill don't know but technically I feel it will get adjusted accordingly