Understood. And basically, from Seed1, the thief will never find out that I also have a Seed2.
Correct.
As far as I know, the BIP39 mnemonic is not working yet on bitcoin core, right?
Correct.
Bitcoin core does not have BIP39 (mnemonic codes) implemented.
While it still creates hierarchical deterministic wallets (BIP32), it does not create a mnemonic code to back it up. The keys have to be backed up by either backing up the master private key or the wallet file.