Can shares be submitted from a demat account that is different from the account that is mentioned in the tender form, provided both the accounts belong to the same person?
The bid number(your 16 digit Client ID with the DP) of your tender form is unique to your buyback application. The RTA will process your buyback based on the bid number and the number of shares one can tender will be fixed per tender form.
You should be able to tender shares in your other demat using the tender form with the DP Client ID of the other broker.