It's not like it's hard to get K and K to cancel each other out using addition. Why are you generalizing from "the inverse of XORing K is XORing K again" to "the inverse of adding K is adding K again"? It isn't, but everyone knows what the inverse of adding K is.

    C1 = P1 + K
    C2 = P2 + K
    C1 - C2 = P1 - P2

And if you can guess part of either plaintext, you'll see the same part of the other plaintext, and know what the key was, exactly the same as for XORing. As somebody else already pointed out, that's because XORing is a variety of addition anyway.