How do Kramnik and Topalov indicate a resignation when they play together? From What is the correct resignation procedure? it is clear that, although FIDE has no special rules governing on how you resign, it is common to either offer your hand or make some sort of exclamation (i.e. "I resign!"). What is important is that your intention to resign is unambiguous.

Now, the rivalry between Kramnik and Topalov has gone to such an extend that each player refuses to shake the other's hand or engage in any kind of pleasantries. Which raises the question - how do they go about resigning when playing each other without creating confusion about their intention?
If I remember correctly, they either just sign the scoresheet or stop the clock first and then sign the scoresheet to indicate that they resigned. There is no handshake, and they do not speak to each other since the WCC2006 controversy.

Here are two examples:
Candidates Tournament 2014, Rd. 13 (Kramnik-Topalov 1-0),
Norway Chess 2014, Rd. 6 (Topalov-Kramnik 1-0).

Addendum. Not asked in the original post, but one interesting (and related) question is also: How do Kramnik and Topalov agree to a draw? Signing the scoresheet or stopping the clock is not appropriate in this case, since the opponent might want to continue playing.

One obvious way is to repeat the position three times. If there is no threefold repetition on the horizon, the players can offer a draw through the arbiter (like in Sofia rules, used, for example, during the WCC 2010).