It is not like sharing a bank account because when it gets hacked, you either never know about it because the bank compensates for the damage before you realize because they have a reason to save their reputation or it is rather easily traceable where the money went and who authorized a transfer.
Yes, I sort of agree with this. I would normally argument against sharing accounts even in families and couples, with the exception the relationship is really trusted. There are such relationships. And even in this case, it could be a good idea to agree on a sort of safety net.
For example, instead of having one key for both, a multisig account made from two wallets with distinct seeds you regularly renew (to avoid it being exposed if one or both parties has better or worse knowledge) could be an interesting idea, because you then know whose key was affected if something goes wrong, and at least the simple "I was hacked!" excuse wouldn't work anymore. This would of course only work for saving/holding purposes, if you move the funds around it would become extremely complicated due to the continuous renewal of the multisig addresses,
Another idea could be that one of both has the keys, but there's a legal agreement that the coins belong to both, and a second key stored e.g. in a bank safe ...
Do you think this could work as an "official recommendation" for the OP list? I guess the details are a bit advanced - but something like "if you share a Bitcoin wallet with your partner, do it only in very trusted relationships, and know exactly what you do and agree on some sort of safety net for the case of a hack." could be a good idea to add.
There are absolutely relationships that you can trust, but this is not a solution to the proof of hack/leakage problem. If one key is affected, you still can't tell whether it happened intentionally or whether the person made a silly mistake, possibly not even being aware of it. If both parties are technically literate, it reduces the possibility for accidental leakage or hack, but it never goes to zero in a 2 out of 2 multisig (or with any other solution for that matter).
The downside I see with this solution is if one party loses access for whatever reason, the funds are frozen. You would also need coordination every time, but as you said it is not meant to be for everyday spending, so coordination may be negligible.
I think a 2 out of 3 multisig is better with a backup key stored in a bank vault or a lawyer or trusted family member. That provides more security to incidents like hardware wallet damage or loss or even a compromised key. In my opinion 2 out of 3 would serve most purposes the best.
As for the rotating seeds idea, I don't think that the ratio of additional security to required effort pays off here. Even if one key is leaked, the chance that the attacker gets access to the second key is basically zero unless a couple or family is a very specific target, but how likely is that? Which actually leads to a very simple but crucial defense layer, which is "don't talk about you owning Bitcoin".
There was recently a topic where they discussed that the seeds could be brute forced or so? I can't exactly remember and whether it referred to older hardware/software wallets. Hardware wallets of today generate excellent entropy, so rotating for the sake of preventing someone from guessing it / brute forcing it doesn't make sense in my opinion. Rotating it because one seed could have been leaked, as I said above, if there is a risk that two specific people are a target, maybe yes. But the circumstances under which an attacker gets access to two seeds to construct a full signature in a 2 out of 2 multisig, I don't know, very unlikely. The problems I see with this is the additional effort, which is generally prone to making mistakes. The more you rotate, the more mistakes can be made. I know that this doesn't apply to technically very literate people, but I am rather referring to the average guy. You have to write down the new backup, verify it's correct, move all your bitcoin, and destroy your old backup securely. That's all prone to human error.
It's not that I think rotating seeds add no value, but does it add value by so much that it offsets the risk of making mistakes for people who are somewhat familiar with all the tech steps? If you add this part to OP, I would perhaps make a distinction like: for average user, for advanced user. And of course if there is a justified suspicion that information could have been leaked because someone made a mistake like uploading it into the cloud or took a foto or whatever, then rotate the seed. But that's the point, advanced users won't make that mistake. You would also have to make sure that no malware is on your devices because then rotating brings no value and if you do it with airgapped machines, well then you are advanced anyway. Maybe a good rule is to rotate when the trust assumption chances, like events in life that indicate a change in the trust environment under which you set up the former arrangement. Or when you know that there is a problem with a hardware wallet of course, things like that. The motivation to rotate should be driven by incidents, not by time window rules. At the end of the day every rotation creates new things that must be done correctly.
One problem you eliminate with a 2 out of 3 is that under normal circumstances, you have a kind of "log" of who accessed the third seed (backup seed) and when, especially when it is escrowed by an attorney or a bank (if the bank keeps access logs, I don't know). In a 2 out of 2 and the couple situation, one partner might know where the other partner stores the seed and you still could not prove who authorized a transaction. And as for the bank vault or lawyer option, I think it could be set up such that the backup seed can only be accessed if both people are present, should both people be alive or able to, but I think that option only applies to larger Bitcoin holdings most of the time. To add an additional layer of security again, the key of the backup seed could be split and stored in two different locations using Shamir's secret sharing for example, but that is really for the super big Bitcoin amounts I think (like you and me).
Some further advice that sounds very simple, but is crucial, are things like labelling backups for what they are like "Bitcoin key" or anything related to that. Label the letter with the information "marriage invitation" and again, never talk about Bitcoin holdings and certainly not about any setup you chose to protect it.
In any case, when setting up multisigs, people should familiarize themselves with secure wallet setups if they own a lot of Bitcoin and that is not only about securely storing the seeds. To recover the wallet as easily as possible, you should store the seed phrase obviously, but also the descriptor, how paths were derived and instructions for recovery. If inheritance becomes relevant, those who find the seeds should also find what kind of wallet it is and how it was set up because that makes recovery much easier. Otherwise it could end up in guess work. However, most wallets today like Sparrow export and import descriptors, which makes it much easier. So maybe that is a reason why people should migrate every once in a while because the state state of the art software usually comes with updates in regards to recovery comfortability.
I think we need more development on the Bitcoin wallet ecosystem because there are still a lot of pitfalls.
