let's say I minted an ERC721 NFT using smart "contract_one" and it was sold by another contract called "contract_two" in which contract the transfer event will trigger?
Transfer event is by default should be emited by
ERC721 smart contract, the standard of ethereum. If your smart
contract B is also emitting some event then it is upto that contract structure and design. So, if, let's say, the smart
contract B is emitting some event like
Sold then both the events are emited in single transaction
Transfer event form ERC721 and
Sold event contract B.