ONLY EVER USE IT OFFLINE! Save this file and the BIP39 word list locally, e.g. on a USB stick.
Boot an OS on a machine that's not connected to the internet. If it's not Tails and can leave a trace, wipe the drive with the OS afterwards, while still offline.
Open this file in a browser.
Click on "Choose file" and open the BIP39 word list. It will be verified against the expected checksum.
Enter two seed phrases and click "XOR". It works with an arbitrary number of seed words (12, 24, ...).
A xor B will be displayed in green. You can verify it by replacing one of the inputs with it and XOR-ing again; the result should be the same as the replaced input.