I logged to icici bank account, the way the fastag account is set up as
Customer ID which will have one or many car association. Each car will have a account and balance against the car. They also have a Fastag Wallet which can be loaded, and if you have more than one Fastag cards or Vehicles associated, you should be able to move the money from Fastag wallet to each vehicle account. Unless you move money into the vehicle account, you will not be able to use the fastag associated with that car.
I have not explored funding the wallet as yet, but I have recharged the Vehicle account with 100 rs, and was able to successfully do it. Now the vehicle account shows balance of 300 which ,means, 200 from the first 499 transaction done and then 100 recharge.
For the wallet though, min of Rs 1000 recharge is mandatory.
I have been allotted a car sticker number. Waiting for the sticker to arrive, don't know when it will arrive.