Tier Nolan suggests a simpler way to implement this, through nTimelock after the bounty's expiration. The proposed symmetrically funded channel requires four 1BTC outputs, which are essentially two single-funded channels. The transaction creates an output paying Alice and Bob 1BTC each. Alice and Bob create TX1, sign it, and share the results. They also create TX2, sign it, and share the results. If one signs the transactions, and the other doesn't, then the signer should spend their inputs to be safe. If TX1 is broadcast, then they can both spend their timeouts to recover their funds, making it safe. Once they both have signed versions of TX1 and TX2, they will broadcast TX1 to initialize the channel. In case TX1 is mutated, they should abort and spend their timeouts to recover their funds. The Lightning-dev mailing list is hosting this discussion.