How do Alice and Bob share their ephemeral private keys simultaneously? If they don't, and let's say Alice first sends Bob her private key. At this point, Bob can broadcast his current commitment transaction, and the previous commitment transaction (as he still has not sent his previous commitment transaction's private key to Alice yet). But Alice can only broadcast her current commitment transaction, as she has sent her revocation private key of her previous commitment transaction to Bob already. This asymmetry is a bit baffling.
If the previous commitment transaction favored Bob, what prevents him from broadcasting it - knowing that Alice cannot penalize him.
My guess of an answer for this is that they share their private keys based on whether value is flowing from Alice to Bob or Bob to Alice. As in, if the previous commitment transaction favors Bob, then he has to share his private key first. Or some such.