Yes I understand what multisig is
But I'm just one party
My sats being shared with a second party wouldn't be self custody
Discussion
each party sends a new signature to the other every time a transaction is made that updates the balance.
this enables either party to unilaterally close the "channel" whenever they want (since they have two valid signatures) and get the sats back out on the mainchain.
Again, that sounds like an on-chain transaction with a fee, but that can't be what you mean because that would mean it's not self-custody