in my past experiences, a little delay at least between players, so that there's enough time when 1 player blocks and the other doesn't see the animation at the same time, but at a small delay. i usually jump attack when they've blocked on their side while i haven't seen the animation yet. if done right, i'll chain with the jump attack, it'll hit with 1 part of the chain and then, the following hit in the chain will generate a block damage value on both clients, which then you can follow up with a combo or if your in trans, with a fast enough jump attack, you can just spam jump attacks.