I've been going through BIP-11, M-of-N Standard Transactions,
A couple of motivating use cases:A wallet secured by a "wallet protection service" (WPS). 2-of-2 signatures required transactions will be used, with one signature coming from the (possibly compromised) computer with the wallet and the second signature coming from the WPS. When sending protected bitcoins, the user's bitcoin client will contact the WPS with the proposed transaction and it can then contact the user for confirmation that they initiated the transaction and that the transaction details are correct. Details for how clients and WPS's communicate are outside the scope of this BIP. Side note: customers should insist that their wallet protection service provide them with copies of the private key(s) used to secure their wallets that they can safely store off-line, so that their coins can be spent even if the WPS goes out of business.
Other than Bisq whose workings include 2-of-2 signatures, are there any WPSs that send the copies of the private keys to their clients? When would they typically do that ( e.g. always at the request or only after filing for bancrupcy)? I don't see much sense of the client having both keys at home from the moment of signing up because it seems to defeat the purpose of 2-of-2 signatures.