Well, either one can send a single message. It's the reply from a different frame that causes trouble. In fact you can have unlimited FTL messages sent in a single reference frame. It's only when you combine multiple FTL reference frames that things get wonky.
But if B can't send an FTL-message to A because he's going to receive an FTL-message from A in what to him is the future, that's your causality violation right there.
Right, I should be more precise with the scenario. A cancelled-message is still a transfer of information. To avoid problems you have to set up the scenario such that only one ship would send a message no matter what. But it can be either ship, and you don't have to know which ahead of time.
