not sure what the goal is for "multi-multi-sig" Not sure either to be honest :) in a lot of ways isn't this part of what sidechains want to do--make the things you can do with a group of sigs more robust Good point, but I guess there are always trade-offs to consider. Also, isn't a a major issue with this the size of the transaction increasing, thus leading to potential problems with block sizes and transaction fees for, say, some type of federation? If you reason you want multi-multi sig is for collaborating (various use cases within the realm of collaboration here: https://utxos.org/uses/ ) it would be more efficient if something like OP_CTV were used, for example, right? Yes. So if I understand correctly, doing multi level multisig is possible today although it doesn't scale in terms block space. And CTV solves that particular problem.