It doesn't matter that much, because if there are two people and they want to make a shared address, then it could be 2-of-2 multisig, 1-of-2 multisig or 0-of-2 multisig. Each of that three cases could be handled by Taproot.

2-of-2 multisig: PTLC as it will be in the Lightning Network.
1-of-2 multisig: Spend by key for the more likely path and spend by TapScript for the less likely. Or spend by single key with some commitment (that could be hidden in a signature), just to know who moved the coins.
0-of-2 multisig: No keys, so just a commitment that anyone can produce (and even attach to someone else, just to reduce costs).

One question. Your diagram (which by the way should not be hosted on Reddit so it doesn't get deleted forever - try Imgur instead) says "secret exponent" next to the final SHA256. Is the SHA256 the secret exponent, or is it taken to the power of a very large unknown number?