The essence of XOR is changing the value of a bit to the opposite. Having two Seeds, we lay them out bit by bit, then change the value of the bits to the opposite in the first Seed only in those places where in the corresponding places of the second Seed there will be a unit.
The only thing is, you will need to recalculate a new checksum for the 12th or 24th last word.
Example: you have two 12-word (132-bit) Seeds, you invert the corresponding first 128 bits, and recalculate the remaining 4 bits of the checksum for the new resulting Seed.
This way you don't have to worry that any online tool will stop working or will work incorrectly, in extreme cases you can restore everything yourself with a piece of paper and a pencil.

Now why are developers of other wallets not rushing to add this function? Adding such a function will require additional memory, which is already not large in modern hardware wallets. The same Trezor created its own SLIP39 standard, but I don't know of any third-party wallet except Keystone that would add this standard. The reason is simple - they don't want to clutter up the memory of their wallets with functions that are not of primary importance.