If you own 2 BTC, there aren't specific Bitcoins that are marked for you.
If you mine 2 BTC, you posses the private keys to two very specific BTC (unspent outputs) on the blockchain. No one else may spend them without the private keys.
If you own 2 BTC in an exchange then it is true that you are unlikely to own any specific 2 BTC but that's an entirely different subject. At that point you don't really own them.
If you own 2 BTC in an exchange then it is true that you are unlikely to own any specific 2 BTC but that's an entirely different subject. At that point you don't really own them.