My reasoning is: if my 12 words are compromised via a physical intrusion, the extension stored in a separate location will guarantee that my wallet is not compromised. Simply splitting the 12 words in half and storing 6 words separately makes a brute force attack easier (I think). That is why I ask.
So you mean you want them to split and have two seeds generated for two owners of the wallet?
The extension is not actually your best choice for this the only solution for this is Multisig wallet with
2 of 2 multisig consists of 2 separate walletsit will generate P2SH addresses after successfully generated a Multisig wallet.
I never heard someone was hacked or brute force by using MultiSig wallet so I'm sure this is the best option you looking for?
The only problem is that the transaction fees from this wallet are pretty expensive compared to the normal wallet but if your purpose is to make a wallet with a co-owner then MultiSig still the best option.
If you want to make a Multisigwallet you can follow this guide below
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https://bitcointalk.org/index.php?topic=5039220.0