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As an example could be XTS with a 512-bit key that treats the key as one 128-bit key and one 348-bit key.

As to why I'm asking it's because XTS runs on one large key that it treats like two keys. Currently, the XTS splits the key in half producing(if we were to use the example above) 2^256 + 2^256(2.3158418e+77) possibilities but if it were to split it differently it could produce 2^128 + 2^348(5.733747e+104) possibilities.

As to why XTS is a thing it's because it is currently the best mode for full drive encryption

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    $\begingroup$ XTS using 2 different keys was actually a harmless mistake due to misunderstanding the original XEX paper. You can use a single key with full security guarantees. AES has no key size greater than 256 bits. $\endgroup$
    – LightTunnelEnd
    Oct 22 at 0:30

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This is really about key management rather than the AES mode itself, if not you need to supply more details.

What's the goal of your suggestion? The threat model? What are the weaknesses you pointed to mysteriously in the question?

Nothing stops you from having a (strange and in depending on the details of use possibly insecure) setup where you store two keys of different length together and use them as your scheme demands. The big question is why.

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    $\begingroup$ I have addressed the issues you pointed out. The weakness I was pointing at in my original question body was the fact that someone could just solve one key at a time. I answered that question myself when I actually put any amount of thought into it. $\endgroup$
    – I don't wanna make an account
    Oct 21 at 23:08

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